Sorry if this has been asked before, but I cannot find anything about it.

Can an ability be used more than once per round (if it does not say as much).

Example: Millennium Falcon is running with C3PO. It is being assailed by three Tie Fighters. On the first attach, C3PO makes his guess. Can he do that for the subsequent two attacks from the other two Tie's? I say yes (flying the Millennium), opponent says "no." You only get to use that ability once per round similar to a pilot's ability.

I cannot find the answer in the rules or the FAQ's.

Thanks!

Abilities, in and of themselves, are only limited to once per opportunity.

Luke Skywalker's pilot ability, for example, is good against every incoming attack, and Han Solo's pilot ability applies to each attack that he makes, if he can make multiple in a turn.

C3P0, however, states "Once per round" on the card itself.

As DraconPyrothayan already says there is not an explicit cap on how often many abilities can be used BUT your chosen example is one of those cases where it is restricted.

If you want a much better example look at Tarn w/ R7 onboard. Tarn's pilot ability can be used to acquire a TL on every ship that targets you although it does move remove the last TL when used. R7's ability allows a TL to be spent to force the Attacker to reroll dice but it IS explicitly restricted to once per round. If you have three TIE Fighters attack Tarn he can take a TL on the first one to shoot at him but then may or may not use R7 against that attack; if R7 is used then Tarn can still pick up (or move if he has one) a TL on the other TIEs when they attack but could not use R7 again until the next round.

Most abilities are once per opportunity, unless otherwise restricted by the text on the card, as in C-3P0s case. One of the most common mistakes is assuming the "once per opportunity" rule means "once per round", which it clearly doesn't. Before applying any "standard" rule or diving into the FAQ looking for an answer, read the card carefully.

Rule of Thumb: If it doesn't say "once per round", you can usually use it whenever an opportunity presents itself or a triggering condition is met.

As the others have said, the cards will specifically say when they can/can't be used. You'll see words like "once per round", "Action:", "Focus", "When you make an attack ..", "If enemy is in your arc ...", "When you make pew-pew noises ...", etc.