321

This number is a composite.

The 321th prime is 2131, which can be written with all the digits of 321. It is the smallest composite number to have this property. [Capelle]

3!-2!+1! and 3!-2!-1! are both prime. [Bajpai]

PrimeGrid has a sub-project called "321 Prime Search" that involves searching for prime numbers of form 3*2^n ± 1. [Prisacariu]