How much have you worked on this? Is he in preschool? It's not uncommon for three year olds not to know all the colors yet. I work with three year olds. Make a game out of learning them......when you're driving...find something..."GREEN" or "RED". Make them OBVIOUS at this age......like a stop sign, etc. This helps them learn to focus, too.
Talk about it with laundry, lunch, at the grocery store, etc. Make a learning experience out of ANYTHING AND EVERYTHING! Do the same with math......how many stop lights until we're home? Then work on subtraction in the same way until you GET home.
You'll be amazed at how well prepared they are for school if you take EVERY experience to interact and help them learn!