I have also found (in my own 3 kids and with d/c kids) that once the third birthday comes and goes, so too does the golden hour(s) of naptime.
Not that you CAN'T get them down for a nap, my 4 yr old will still sleep in the afternoon, sometimes. But I don't expect them to sleep everyday anymore. It's not unreasonable, though, to have quiet time. A movie or some cartoons, coloring, puzzles, independent play with cars/trains, etc. As long as it is QUIET and relatively calm, it's fine with me. I have also taken the non-nappers outside for some extra play, just bringing the baby monitor or checking every 5 min or so.
Set up the expectation that it is time to "play gently and quietly on your own" and you should still be able to have a little bit of time to yourself.
waterloo: i hear ya! when i only have my 5 yr old d/c kid, and the little ones are napping or something, he mopes around like his dog just ran away!! I feel like I should always be "hey, hey, hey... let's do this now!!" but he should be able to play on his own, too, no??