The minimum characters is 4

The only reason they got 3 is due to the following:
Bought a name change and the staff member allowed him/her to have 3 characters
Managed to get lucky through registration and it bugged out allowing them to have 3 letters.(back in 2010, you could got _ with your name, due to a bug happening iirc and that was disabled around 200

Unless they changed it and allow people to have 3 characters now.