No three letter namechanges. The minimum length for a new username is 4 characters, maximum length is 12.

If the name is currently taken, and:
- it's an account with less than 500 qi/100 posts/etc and has been inactive for 3 years, it's available.
- it's an empty account (0 qi/tc/etc) and has been inactive for 3 months, it's available.

Otherwise, the name is not available.