Three is the first odd prime number.

A number is divisible by 3 if the sum of its digits is divisible by 3.

Also:

• 3 is the number of non-collinear points needed to determine a plane and a circle
• 3 is a triangular number, being the only prime triangular number
• 3 is a lucky number
• A triangle has 3 sides
• A tetrahemihexahedron has 3 squares
• 3 is a Fermat prime 
• 3 is a Mersenne prime
• 3 is a Sophie Germain prime
• 3 is a Fibonacci number
• 3 is a palindromic number.