If there was a 3rd branch for thief, bowman or pirate...

Pirates ARE thieves though... You'd have a better time arguing that Ninjas, despite being ROGUES are not THIEVES.

I would say Gunners should have been 3rd class for Archers, and the fisticuffers should have been 3rd class for Thieves. Archers are marksmen, so it would make sense for a person using a pistol to join their ranks, and a thief beating enemies with his "bare hands" wouldn't be outside the realm of logic either.

ps I don't think a ballista user would be any good... do you know how big that pomegranate is? Think of a siege catapult. Think of how big that is. Now imagine A pineappleING CROSSBOW THAT SIZE. That's what a ballista is.

Although Infighters could pass as thieves, how exactly do Gunslingers fall under "Archer"? o_o
Yea, they use Dex, but that's easily changed.
The only similarity is that they're ranged.