Kid birthday parties, help!

I’m facing a new challenge as a mom. My daughter now has friends and they have birthdays. I need some social protocol guidance. She’s been invited to 3 birthdays in 3 weeks from people at martial arts. I assume it’s rude to go with out a gift but how much do you spend and how do you know what they would like? My daughter is 3. The first 2 were for 4 yr olds so we could come up with gifts but now a 9yr old party has me stumped. I suspect this is going to be an ongoing problem so it’s not just about what to get her. It’s also about how do I figure out what to do for the rest of the wonderful kids that invite us.