3

The use of three lines to denote the number 3 occurred in many writing systems, including some (like Roman and Chinese numerals) that are still in use. That was also the original representation of 3 in the Brahmic (Indian) numerical notation, its earliest forms aligned vertically.[1] However, during the Gupta Empire the sign was modified by the addition of a curve on each line. The Nāgarī script rotated the lines clockwise, so they appeared horizontally, and ended each line with a short downward stroke on the right. In cursive script, the three strokes were eventually connected to form a glyph resembling a ⟨3⟩ with an additional stroke at the bottom: ३.

The Indian digits spread to the Caliphate in the 9th century. The bottom stroke was dropped around the 10th century in the western parts of the Caliphate, such as the Maghreb and Al-Andalus, when a distinct variant ("Western Arabic") of the digit symbols developed, including modern Western 3. In contrast, the Eastern Arabs retained and enlarged that stroke, rotating the digit once more to yield the modern ("Eastern") Arabic digit "٣".[2]

In most modern Western typefaces, the digit 3, like the other decimal digits, has the height of a capital letter, and sits on the baseline. In typefaces with text figures, on the other hand, the glyph usually has the height of a lowercase letter "x" and a descender: "". In some French text-figure typefaces, though, it has an ascender instead of a descender.

A common graphic variant of the digit three has a flat top, similar to the letter Ʒ (ezh). This form is sometimes used to prevent falsifying a 3 as an 8. It is found on UPC-A barcodes and standard 52-card decks.

3 is the second smallest prime number and the first odd prime number. It is the first unique prime, such that the period length value of 1 of the decimal expansion of its reciprocal, 0.333..., is unique. 3 is a twin prime with 5, and a cousin prime with 7, and the only known number ${\displaystyle n}$ such that ${\displaystyle n}$! − 1 and ${\displaystyle n}$! + 1 are prime, as well as the only prime number ${\displaystyle p}$ such that ${\displaystyle p}$ − 1 yields another prime number, 2. A triangle is made of three sides. It is the smallest non-self-intersecting polygon and the only polygon not to have proper diagonals. When doing quick estimates, 3 is a rough approximation of π, 3.1415..., and a very rough approximation of e, 2.71828...

3 is the first Mersenne prime, as well as the second Mersenne prime exponent and the second double Mersenne prime exponent, for 7 and 127, respectively. 3 is also the first of five known Fermat primes, which include 5, 17, 257, and 65537. It is the second Fibonacci prime (and the second Lucas prime), the second Sophie Germain prime, the third Harshad number in base 10, and the second factorial prime, as it is equal to 2! + 1.

3 is the second and only prime triangular number, and Gauss proved that every integer is the sum of at most 3 triangular numbers.

3 is the number of non-collinear points needed to determine a plane, a circle, and a parabola.

Three is the only prime which is one less than a perfect square. Any other number which is ${\displaystyle n^{2}}$ − 1 for some integer ${\displaystyle n}$ is not prime, since it is (${\displaystyle n}$ − 1)(${\displaystyle n}$ + 1). This is true for 3 as well (with ${\displaystyle n}$ = 2), but in this case the smaller factor is 1. If ${\displaystyle n}$ is greater than 2, both ${\displaystyle n}$ − 1 and ${\displaystyle n}$ + 1 are greater than 1 so their product is not prime.

A natural number is divisible by three if the sum of its digits in base 10 is divisible by 3. For example, the number 21 is divisible by three (3 times 7) and the sum of its digits is 2 + 1 = 3. Because of this, the reverse of any number that is divisible by three (or indeed, any permutation of its digits) is also divisible by three. For instance, 1368 and its reverse 8631 are both divisible by three (and so are 1386, 3168, 3186, 3618, etc.). See also Divisibility rule. This works in base 10 and in any positional numeral system whose base divided by three leaves a remainder of one (bases 4, 7, 10, etc.).

Three of the five Platonic solids have triangular faces – the tetrahedron, the octahedron, and the icosahedron. Also, three of the five Platonic solids have vertices where three faces meet – the tetrahedron, the hexahedron (cube), and the dodecahedron. Furthermore, only three different types of polygons comprise the faces of the five Platonic solids – the triangle, the square, and the pentagon.

There are only three distinct 4×4 panmagic squares.

According to Pythagoras and the Pythagorean school, the number 3, which they called triad, is the noblest of all digits, as it is the only number to equal the sum of all the terms below it, and the only number whose sum with those below equals the product of them and itself.[3]

There are three finite convex uniform polytope groups in three dimensions, aside from the infinite families of prisms and antiprisms: the tetrahedral group, the octahedral group, and the icosahedral group. In dimensions ${\displaystyle n}$ ⩾ 5, there are only three regular polytopes: the ${\displaystyle n}$-simplexes, ${\displaystyle n}$-cubes, and ${\displaystyle n}$-orthoplexes. In dimensions ${\displaystyle n}$ ⩾ 9, the only three uniform polytope families, aside from the numerous infinite proprismatic families, are the ${\displaystyle \mathrm {A} _{n}}$ simplex, ${\displaystyle \mathrm {B} _{n}}$ cubic, and ${\displaystyle \mathrm {D} _{n}}$ demihypercubic families. For paracompact hyperbolic honeycombs, there are three groups in dimensions 6 and 9, or equivalently of ranks 7 and 10, with no other forms in higher dimensions. Of the final three groups, the largest and most important is ${\displaystyle {\bar {T}}_{9}}$, that is associated with an important Kac–Moody Lie algebra ${\displaystyle \mathrm {E} _{10}}$.[4]

The trisection of the angle was one of the three famous problems of antiquity.

• The Roman numeral III stands for giant star in the Yerkes spectral classification scheme.
• Three is the atomic number of lithium.
• Three is the ASCII code of "End of Text".
• Three is the number of dimensions that humans can perceive. Humans perceive the universe to have three spatial dimensions, but some theories, such as string theory, suggest there are more.
• Three is the number of elementary fermion generations according to the Standard Model of particle physics.
• The triangle, a polygon with three edges and three vertices, is the most stable physical shape. For this reason it is widely utilized in construction, engineering and design.[6]
• The ability of the human eye to distinguish colors is based upon the varying sensitivity of different cells in the retina to light of different wavelengths. Humans being trichromatic, the retina contains three types of color receptor cells, or cones.
• There are three primary colors in the additive and subtractive models.
• In particle physics, each proton or neutron is composed of three quarks.
• In physics, three-body problems have no general closed-form solution, unlike two-body problems.

• Philosophers such as Aquinas, Kant, Hegel, C. S. Peirce, and Karl Popper have made threefold divisions, or trichotomies, which have been important in their work.
• Hegel's dialectic of Thesis + Antithesis = Synthesis creates three-ness from two-ness.

Symbol of the Triple Goddess showing the waxing, full and waning Moon

Many world religions contain triple deities or concepts of trinity, including:

• The Hindu Trimurti
• The Hindu Tridevi
• The Triglav (lit. "Three-headed one") was the chief god of the slavs
• The Three Jewels of Buddhism
• The Three Pure Ones of Taoism
• The Christian Holy Trinity
• The Triple Goddess of Wicca

The Shield of the Trinity is a diagram of the Christian doctrine of the Trinity.

• In American and Canadian football, a field goal is worth three points.
• In association football:
  • For purposes of league standings, since the mid-1990s almost all leagues have awarded three points for a win.
  • A team that wins three trophies in a season is said to have won a treble.
  • A player who scores three goals in a match is said to have scored a hat-trick.
• In baseball:
  • A batter strikes out upon the third strike in any single batting appearance.
  • Each team's half of an inning ends once the defense has recorded three outs (unless the home team has a walk-off hit in the ninth inning or any extra inning).
  • In scorekeeping, "3" denotes the first baseman.
• In basketball:
  • Three points are awarded for a basket made from behind a designated arc on the floor.
  • The "3 position" is the small forward.
• In bowling, three strikes bowled consecutively is known as a "turkey".
• In cricket, a bowler who is credited with dismissals of batsmen on three consecutive deliveries has achieved a "hat-trick".
• In Gaelic games (Gaelic football for men and women, hurling, and camogie), three points are awarded for a goal, scored when the ball passes underneath the crossbar and between the goal posts.
• In ice hockey:
  • Scoring three goals is called a "hat trick" (usually not hyphenated in North America).
  • A team will typically have three forwards on the ice at any given time.
• In professional wrestling, a pin is when one holds the opponent's shoulders against the mat for a count of three.
• In rugby union:
  • A successful penalty kick for goal or drop goal is worth three points.
  • In the French variation of the bonus points system, a team receives a bonus point in the league standings if it wins a match while scoring at least three more tries than its opponent.
  • The starting tighthead prop wears the jersey number 3.
• In rugby league:
  • One of the two starting centres wears the jersey number 3. (An exception to this rule is the Super League, which uses static squad numbering.)
• A "threepeat" is a term for winning three consecutive championships.
• A triathlon consists of three events: swimming, bicycling, and running.
• In many sports a competitor or team is said to win a Triple Crown if they win three particularly prestigious competitions.
• In volleyball, once the ball is served, teams are allowed to touch the ball three times before being required to return the ball to the other side of the court, with the definition of "touch" being slightly different between indoor and beach volleyball.

• A number of film versions of the novel The Three Musketeers by Alexandre Dumas: (1921, 1933, 1948, 1973, 1992, 1993 and 2011).
• 3 Days of the Condor (1975), starring Robert Redford, Faye Dunaway, Cliff Robertson, and Max von Sydow.
• Three Amigos (1986), comedy film starring Steve Martin, Chevy Chase, and Martin Short.
• Three Kings (1999), starring George Clooney, Mark Wahlberg, Ice Cube, and Spike Jonze.
• 3 Days to Kill (2014), starring Kevin Costner.
• Three Billboards Outside Ebbing, Missouri (2017), starring Frances McDormand, Woody Harrelson, Sam Rockwell.

• Cube (algebra) – (3 superscript)
• Thrice
• Third
• Triad
• Trio
• Rule of three
• List of highways numbered 3

• Wells, D. The Penguin Dictionary of Curious and Interesting Numbers London: Penguin Group. (1987): 46–48

Look up three in Wiktionary, the free dictionary.

Wikimedia Commons has media related to 3 (number).

• Tricyclopedic Book of Threes by Michael Eck
• Threes in Human Anatomy by John A. McNulty
• Grime, James. "3 is everywhere". Numberphile. Brady Haran. Archived from the original on 2013-05-14. Retrieved 2013-04-13.
• The Number 3
• The Positive Integer 3
• Prime curiosities: 3