3PO is fussy, panicky, self important etc but in the OT he's never dumb. In ESB he repeatedly tries to tell Han vital information and gets ignored, in an ESB deleted scene he sets a trap for Stormtroopers (But that doesn't count), he tries to talk sense into Chewbacca who is intent on killing Lando etc. In ROTJ he negotiates with the Ewoks for their help which arguably has more to do with the victory over the Empire than anything Leia does (Again with Han undermining him).

In the PT he was a clown, worse than that he was a fourth wall defying comedian delivering bad puns in AOTC. In TPM and ROTS he does jack sh*t.