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| Cardinal | 3 three |
| Ordinal | 3rd third |
| Numeral system | ternary |
| Factorization | prime |
| Roman numeral | III |
| Prefixes | tri-, tria- (Greek) tre- (Latin) |
| Binary | 11 |
| Octal | 3 |
| Duodecimal | 3 |
| Hexadecimal | 3 |
3 (three) is the natural number following 2 and preceding 4. Prefixes for 3 include tri- (Greek) and tre- Latin.
Three is often the largest number written with as many lines as the number represents. Even the Romans tired of writing 4 as IIII, and even today in Chinese, 3 is written as three lines. This was the way the Brahmin Indians wrote it, and the Gupta made the three lines more curved. The Nagari started rotating the lines clockwise and ending each line with a slight downward stroke on the right. Eventually they made these strokes connect with the lines below, and evolved it to a character that looks very much like a modern 3 with an extra stroke at the bottom. It was the Western Ghubar Arabs who finally eliminated the extra stroke and created our modern 3. (The "extra" stroke, however, was very important to the Eastern Arabs, and they made it much larger, while rotating the strokes above to lie along a horizontal axis, and to this day Eastern Arabs write a 3 that looks like a mirrored 7 with ridges on its top line).
Three is the second smallest prime number (after two); the next prime number is five. Three is the first Fermat prime (220 + 1) as well as the first Mersenne prime (22 - 1). 3 is the second Mersenne prime exponent. 3 is also the first lucky prime.
It is also a factorial prime (2! + 1), and a unique prime due to the properties of its reciprocal.
Three is the second triangular number.
Three is the fourth Fibonacci number and the third that is unique. It is also a Lucas number.
Three is the fourth open meandric number.
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 the sum of its digits in base 10 is divisible by 3.
Many human cultures have given the concept of three-ness symbolic meanings. The Holy Trinity in Christian doctrine (or trinity in general), is God both a single entity and three entities, the Father, the Son and the Spirit.
The process of synthesis in Hegelian 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. There is also the concept of Trimurti in Hindu tradition.
Three (三, formal writing: 叁, pinyin san1) is considered a good number in Chinese culture because it sounds like the word "alive" (生 pinyin sheng1), compared to four.
A group of three is often called triad.
See also: List of famous trinities, trios, triplets, or threesomes
Triacid, Triaminic, Triamcinolone, Triazine, Triazole, Tribromoethanol, Trichloroethylene, Trichlorfon, Trifluralin, Triglyceride, Triglycerophosphate, Triphosphopyridine nucleotide
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