I think that, at three months, babies should be sleeping about 10 hours at night (my five month old isn't doing this yet, though

. From reading other posts here, though, it seems like many babies sleep from 7 PM until 7 AM. In the General Sleep thread, there's a post that gives sleep guidelines based on age.
My guess would be that you shouldn't feed him when he wakes at 3:30 or 4:00. My daughter wakes at about 4 AM and I used to feed her, assuming she was hungry. But then when I fed her again at 7 AM (which is when we begin our day on EASY), she wasn't hungry...... so, I decided she wasn't hungry when she woke at 4:00. Now I use shh/pat when she wakes during the night and wait until 7 AM to feed her.
If your son is getting 25 to 28 ounces (if you're bottle feeding) during the day, then he probably isn't hungry when he wakes at 3:30.
If you want your son to wake at 6:00 or 6:30, it's probably fine, since he goes to sleep by 7 at night..... but if you want him to sleep later, you might try putting him down at 7:30. In Tracy's sample 3 hour EASY routine, she suggests a 7 AM wake time and 7:30 PM bedtime.
HTH