I LOVE almost Halloween birthdays! My oldest was born on 10/25.
The kids seem to love dressing up, trying to bite apples hung from a string (as opposed to bobbing in water), making masks from paper plates and trying to toss home-made bean bags into a bowl or jar (you could disguise the jar to look like some Empire target, and the bean bags could be proton missiles).
Then, there's pin the *** on the ***. Could be "tail" and "donkey", or "helmet" on "Vader", the kids will love it!
I learned early that about 3 games/party are all they can handle, they'd rather play with each other (kids age = number at party, so five guests plus party guy for oldest's party).
Instead of guest bags, I make party poppers for the kids. It's different, and fun, and a blast at the table. I use empty toilet paper rolls, cut a tissue paper crown for each, put in some silly joke (like "What do you call a hot dog and a root beer on Hallowe'en? Frank and Stein!) (a different one for each, obviously), a balloon, a lolly, and a small prize. Then I wrap the whole thing in theme wrapping paper, tie the ends in curling ribbon and fray the ends of the paper with scissors. Kids that age don't mind that there's no loud popper inside, although you can usually get a string popper left over from the 4th if you hurry and you really want one (I don't use them at all for kids).
I also keep the party short so there's no possibility of boredom, no longer than an hour and a half.
For someone who's three, I still give family parties. I don't start with "friend" parties until "real" school. That way, there's no hurt feelings if there's a classroom full of pre-schoolers who might be left out, and neither you nor the child is overwhelmed by a too-large party of small people. If he really wants his "own party", and there's a way to do it, just "invite" his brother and set up a special, brothers-only game time. Maybe make it a livingroom sleep over, with a family special meal (maybe Mr. 3 loves Mac'n Cheese? Pizza? Chicken?) followed by Candyland or Twister (OK, I LOVE to watch a 3 year old try to play Twister...), cake 'n ice cream, gifts and party bags or poppers, and a special movie and sleeping bags on the living room floor.
Well, guess I'm long winded! Anyway, there's my thoughts. Hope it's somewhat helpful!
Jamie