321

321 = 3 x 107.

321 is a Delannoy number. The Delannoy numbers are the number of lattice paths from (0, 0) to (b, a) in which only east (1, 0), north (0, 1), and northeast (1, 1) steps are allowed.

The 321st prime is 2131, which can be written with all the digits of 321. It's the smallest composite number to have this property.

321 is generated by the following sieve: Keep the first number, drop every second number, keep first, drop every third, keep first, drop every fourth, and so on.

Telephone area code 321 serves Brevard County, Florida.

Source: Wolfram MathWorld