Waiting until the turn to raise is a higher percentage bluff, IMO. That's what the fish would do with a 3, so they respond to it better. Activity on the flop is usually read as a made hand, but not the obvious hand. He probably thought you had 5's or 7's there because of the preflop limp. The only other thing that fits a tight player would be A3s.

Then again, you just have to show some players a hand. Bluffing is often a waste of time and chips at microlimits.