1st birthday i did theme, 2nd (just a couple weeks ago) i was trying to do chuckEcheese, but money was an issue so we just went to the park with a friend (and their child) and then ice cream and home for family gathering (no more then are usually around the house throughout the week, but we never are "together" for dinner like that day). as for future birthdays i plan on doing mostly themes until my daughter can ask, or tell me, specifically what she would like. if she wants a character (that's available) i wouldn't deny her, but for the younger ages i think its a little too expensive to go too big (other than 1st birthday) plus i've been at a few parties for younger children where some that were invited, and even sometimes the birthday child themselves, get frightened seeing their favorite character HUGE and in person (usually it was barney lol). but when she's older and if she asks for one, that would be fine with me.
since most (unlike me) plan months in advance on what they'll do for a party...if it's for your 12 yr old, i would get her opinion on what she would like...try to just bring it up casually, and get a few ideas, and do it months before you would do anything. if you want it to be a surprise, after you get the ideas, never mention it again, or ignore/change the subject if brought up again. or you could just have her involved in the whole thing (i know as a child past 8yrs, i almost always hated what my mother chose if she didn't get any kind of input from me on my preferences and always felt bad hating it and wishing that she just didn't waste her money and didn't bother doing anything). as for your 3 yr old, do themes, maybe "outings" like chuckEcheese, or mcdonalds (not 100% sure if they still do parties), or even theme parks or zoos (a friend did one at turtle back zoo a couple yrs back)...go by what your child likes, and of course what you can afford. good luck on whatever you're planning.