L.T.
How about a basketball hoop? We have an adjustable height one that the kids get a lot of use out of.
Little people playsets are fun and spark imagination.
Playdoh and accessories - my kids will play with it for hours!
Puzzles.
Cranium Hullaballo gets the kids moving and acting out commands.
Crayola Color Wonders products - kids can create artwork without the mess of spilled paints, etc.
Candyland Castle - a fun, quick matching game that we played over and over and over.
Magnadoodle - another favorite that has been a life saver on long car rides and in restaurants.
Instruments.
Online - pbskids.org has a variety of games. Although at "just turning 3" there were only 3 or 4 games they could understand enough to play. There is a Christmas website that is fun. I don't know if your son can play the games himself but you could help. Go to www.northpole.com. I think they had one game my kids especially liked at that age. It was either decorating a tree or a snowman.