Robyn,
If your son was 4 1/2 and heading to kindergarten in the fall, maybe I'd be concerned, but at three, it's no big deal. All kids learn at different paces. For example, my son is 2 years, 4 months. He can sing the alphabet, he can count to 16 or so, and he can sing nursery rhymes. But he has almost zero spontaneous vocabulary outside of that. For all you know, the Mom next door sits her kid in front of Sesame Street all day long, and the kid memorized it from there. Or she's some kind of academics fanatic who aims flash card at the poor thing.
Because my son is getting ready for pre-school in September, I've been dealing with a lot of this. I've researched on the internet, talked to other Moms, even interviewed preschool teachers. What I've come up with is that pre-school should be primarily a social experience, especially the three year old programs. What I am looking for out of next year for my son is for him to be able to separate confidently from me (I'm a SAHM), to follow direction from another authority figure, to feel confident and comfortable in a classroom environment, and to learn to make friends with other kids. That seems like a pretty tall order for a three year old!
If you want your son to get more traditional pre-reading skills under his belt, work on them through his play time. Teach colors and how to mix colors and all with some finger paint. Get a memory game, and let him count the number of matches at the end. Let him watch Sesame Street on the weekends, or get him one of those electronic learning games for pre-schoolers for Christmas. The kids love to play them, it is a viable teaching tool, and there's no pressure or expectations. Your son is going to deal with grades and testing and demands for certain levels of performance his WHOLE life. Does it have to start at 3?