Well there are certain advantages T3 has over the other droids. T3 is the only one with a self-sustaining module, an unlimited-use special weapon (shock arm), and regenerative shield. Those essentially make it so that T3 could always take punishment and be able to come back for more.
Goto and HK-47 may be able to fight better than T3, but they depend upon having plenty of repair kits and replacement weapons and shields. In the early stages of the game, T3 really has the advantage, but at the later stages; that +3 regeneration module and shock arm become less significant to the outcome of a fight. The regenerative shield only works when T3 advances to level 13.