Its all about the principle as far as I'm concerned.

Its not a BBQ gathering at a friends place. Its a combined birthday for 3 separate children (and IMO you should have been invited by only one of the girls thus only brining one present, a friend of mine who has twins does this and she gets annoyed if everyone brings them both presents) and they are to bring food to share? Sorry but thats not good etiquette at all. I'd do it, but I'd also think they are extremely cheeky.

I'm all for generosity and the like...

Which leads me to part 2 here.

Is there a reason the birthdays are combined? Has something happened in one of their families you might not know about? A death, loss of income etc Are the mother banding together to help another? Coz that to me seems like that would be the only reason one would do this so that a child doesn't miss out on a super birthday, and maybe the other parents couldn't afford to do a joint birthday for this one child? I know I am totally making assumptions, but it could be charity.