you could try something fun and educational like toys that name colors and count depending on the boys developement if he can speak well and knows colors and counting try ac toys with adult supervision or buying legos or lincoln logs boys love to build and construct. toy tool sets are also a good choice.
i go with the little pads that have color and counting and animal identification games. they love them and have no idea they're learning. they may stretch the 3 yr. old's gift budget,but it's so worth it. anyway,enjoy his party!