3

Number
$3$ (three) is:


 * The second triangular number after $1$:
 * $3 = 1 + 2 = \dfrac {2 \left({2 + 1}\right)} 2$


 * The second prime number after $2$


 * The first odd prime


 * The first prime number of the form $4 n + 3$


 * The first Mersenne prime


 * The first Fermat prime


 * The first prime number which is the first of a pair of twin primes


 * The first of an arithmetic progression of primes: $3, 5, 7$


 * The sum of the first two factorials: $3 = 1! + 2!$

Also see

 * Trisecting the Angle
 * Three Points Describe a Circle
 * Three Regular Tessellations
 * Integer is Sum of Three Triangular Numbers
 * Integer as Sum of Three Squares
 * Divisibility by 3
 * Smallest Magic Square is of Order 3