C-3PO was designed as a protocol droid intended to assist in etiquette, customs, and translation, boasting that he is "fluent in over six million forms of communication". (wikipedia)
so he gets a very special leader ability:
„if c-3po is in the leader spot, he activates ALL LEADER ABILITIES of the other 4 heroes in his team“ (zeta)
and a little help for his droid-friends:
„as long as c-3po is alive, target lock is locked, until he dies.“ (zeta)