At 3 my boys really enjoyed the Galactic Heroes (Star Wars). This is of course more exciting if he knows what Star Wars is. They have expanded the line of squatty little action figures so that if they aren't into SW there is spiderman, gi joe, etc.
The Imaginext play sets are FANTASTIC! They have several different ones, a castle, bat cave, etc. A set with some additional characters would go over well.
Three is also an age that many kids get Leapsters. In our family though, my boys grew up around video games and at 3 my youngest wanted a Nintendo DS, didn't have any interest in a Leapster.
Hope this helps.