Three

''For the year AD 3, see 3.

Three (3) is the natural number following two and preceding four.

Table of contents

1 Properties of three 2 Three-ness in human culture 3 More references to groups of three 4 Chemicals 5 Other references to three

Properties of three

• Three is the second smallest prime number, and the only one divisible by 3, the next is five.
• Three is the second triangular number.
• Three is the first Fermat prime.
• Three is a Fibonacci number (And it is the third different one, too!).
• Fractions with 3 in the denominator have a single digit repeating sequences in their decimal expansions, (.000…, .333… or .666…)
• A natural number is divisible by three if and only if its decimal digit sum is divisible by 3.
• The base-3 numeral system is the ternary system.

Three-ness in human culture

Many human cultures have given the concept of three-ness symbolic meanings:

• The Holy Trinity in Christian doctrine (or trinity in general)
• The process of synthesis in Hegelianian dialectic creates three-ness from two-ness
• The three Doshas (weaknesses) and their antidotes are the basis of Ayurvedic medicine in India
• The three Gunas underlie action, in the Vedic system of knowledge
• The Trimurti in Hindu tradition
• Three (三 pinyin san1) is considered a good number in Chinese culture because it sounds like the word "alive" (生 pinyin sheng1), compare to four

More references to groups of three

• 3-D
• Three-decker - a ship of three decks
• Three-spine stickleback - a type of fish
• Three-toed woodpecker
• Threepence - three-penny coin
• Threesome - trio or else
• Thrice - three times
• Triad - a group of three
• Trialogue - conversation among three
• Triangle - polygon with three sides
• Triangulation
• Triangulum and Triangulum Australe - constellations
• Triarchy - government of three
• Triassic Period - geologic time
• Triathlete - contestant in Triathlon
• Triathlon - contest in three sports
• Tribrach - metrical foot, in poetry
• Triceps - a muscle
• Triceratops - a dinosaur
• Trichotomy - division into three parts
• Trichroism - property of minerals such as tanzanite, andalusite and iolite
• Trichromatism - of three-color technology or perception
• Triclinium - a couch to three sides of table
• Tricolor - three-color flag, as in France, Romania, Moldova
• Tricorn - of three corners, e.g., a hat with "tricorned brim"
• Tricuspid - of three cusps
• Tricycle - (bicycle × 1½)
• Tridactyl - of three toes
• Trident - of three teeth or prongs
• Triennium - three-year time
• Trierarch - captain or manager of a trireme
• Trifid - cleft into three
• Trifocal - a lens (e.g., of eyeglasses) with three focal lengths
• Trifoliate - a leaf structure
• Triforium - a certain gallery in church
• Trifurcate - threeforked or threestemmed
• Trigeminus - trigeminal nerve
• Triglyph - an ornament
• Trigon - triangular harp
• Trigonometry - a branch of mathematics
• Trigram - I Ching thing
• Trigraph - three-letter combination
• Trihedron - afigure of three meeting planes
• Trihybrid - a kind of hybrid
• Trilateral - of three sides or parties
• Trilingual - able of or expressed in three languages
• Triliteral - a word or root of three letters
• Trilithon - prehistoric three-stone structure
• Trillion - 10¹² (American) 10¹⁸ (British)
• Trilobate - of three lobes
• Trilobite - a marine arthropod
• Trilogy - three works on a theme under the same authorship
• Trio - three musicians or three actors
• Tripartition - division into three parts
• Tripod - a three-legged structure
• Trireme
• Triumvirate - government of three
• Trivium - grammar+logic+rhetoric ("three roads")
• Troika

Chemicals

Triacid, Triaminic, Triamcinolone, Triazine, Triazole, Tribromoethanol, Trichloroethylene, Trichlorfon, Trifluralin, Triglyceride, Triglycerophosphate, Triphosphopyridine nucleotide

Other references to three

• "In Memory of 3" - a phrase about NASCAR driver Dale Earnhardt
• No. 3 - 1997 Korean gangster comedy written and directed by Neung-han Song
• 3 - a rock band formed by Keith Emerson, Robert Berry and Carl Palmer
• Three Kingdoms
• Three Gorges
• Three-letter abbreviations and acronyms
• Three links
• Three of a kind - in poker
• Three Principles of the People
• Three Represents
• Three's Company - an American sitcom
• Three Years of Natural Disasters

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See also: one, two, three, four, five, integer, list of numbers.