Hi. Just come across this thread. I am sure you must have started planning your son’s third birthday. If you still don’t want a commercial theme, I would recommend you to organize a soccer party for him. Ask the boys to dress up as players and the girls to dress up as cheerleaders. Buy soccer themed party supplies such as cups, plates, napkins, balloons, etc. Make cake looks special by using soccer ball pick candles. You can get these http://www.mybirthdaysupplies.com/boys-birthday-party-supplies.php from Mybirthdaysupplies.com.
I think barnyard or jungle safari would be a great theme. You can also have a simple balloon theme party.