3

3 is a prime number. 3 is also a Sophie Germain prime because 2 x 3 + 1 = 7, which is a prime.

3 is the second triangular number, or the sum of the first two positive integers: 3 = 1 + 2.

3 is the fourth Fibonacci number: 1, 1, 2, 3, 5, 8, 13, 21, . . .

3 is the second Lucas number: 1, 3, 4, 7, 11, 18, 29, . . .

3 has a unique representation as a sum of three squares: 3 = 1² + 1² + 1².

3 is the number of integer partitions of 3.

3 is the number of integer partitions of 5 into distinct parts.

3 and 5 form a twin prime pair.

Tri- means three. So triangles have three sides, a tripod has three legs, and the dinosaur triceratops had three horns. The French flag is a tricolore because it has three colors.

Three-dimensional means that something has length, width, and depth.

The letters A F H K N Y Z are all made up of three lines.

The trillium, Ontario's provincial flower, has three petals.

Source: R. Phillips. 1994. Numbers: Facts, Figures and Fiction. Cambridge University Press.