my daughter went to every preschool party she was invited to last year, except one because of a family party. some kids never went, some only went when able.
what you first need to realize is your reason of "not wanting to socialize" should not even be a reason. because you are antisocial, is it fair to make your child antisocial as well? dont you want your child to have friends and be included? you dont have to talk to people if you dont want to. at 3, chances are your child will need your participation in some way and you arent going to be just sitting around. and you should want to see the children you son is around. its good to see the families of other children, it gives you an idea of who you might feel is a good playdate, or maybe one you want to avoid. its not about YOU socializing, its about being an INVOLVED parent!!!
if you really cant afford the presents, then so be it, but i have gotten presents probaly $5-7. there is no rule that you have to get some extravagent gift. i actually would have prefered a coloring book and markers compared to toys.
and as for your sons party, i believe its very important to teach children at a young age that is about giving, not receiving. your son should never be going to a party because he expects them to come to his in return. he can easily understand he is having a family party instead of one with friends. i dont that needs to be a consideration.
so i guess i dont think any of your reasons are sufficient, lol. i really dont mean to come across harsh. its just that im not truley outgoing, but i would never let my personality trait affect my daughter and how she lives. its very important to realize preschool is about teaching your child to socialize with others, to learn the world does not revolve around them, and sharing with others. to deny your child an experience because you will feel uncomfortable is unfair, but also will instill a negative(antisocial is not positive) trait in him as well. that would actually be one of the reason i would go.
and it will be fun to watch your son interact with classmates. bring a camera, im sure there will be lots of cute things he does you will enjoy. i would say it would be more in his benefit to tell him about the party, whether you go or not. children are already talking about it, telling the child if they are coming. your son may think he was never invited in the first place. and if he would beg you to go, you need to be the adult and be honest with him, you shouldnt lie about this.
just go, and realize the party is not expected to be a good time for YOU, its for 3-4 year olds. and remember, you keep making a point about not knowing anyone. well...... dont you think this will be a good way to get to know people??? then, you wont be able to say "i dont know anyone" as an excuse not to go the 2nd, and 3rd party.
and just so you know, FATHERS are very involved these days. whether married or divorced, i would say every party i go to have 50% fathers, times have changed. you could easily decide to switch every other party. LET US ALL KNOW HOW IT WENT!!!