To be honest since dealing with more parties these days, I would take the Maccas party over some that DD1 has been to lately.
Exactly that! A lot of play centres food is absolutely rubbish compared to Maccas. We went to a party at our local roller-rink a couple of weeks ago and I was astounded at the rubbish that was served up - even my DD who enjoys the odd bit of junk food barely touched it!

I do find a Maccas party at 3 a bit young - but how old is the other child turning? And who knows the family cirumstances behind their reasonings for having it.

Personally I would send my kid along and I wouldn't over think it. It will be what it is and the outcome (ie the new found knowledge of Macca's) will be as bad or as good as you let it to be...Does that make sense? In otherwords if you make a fuss about not eating Macca's etc then their is a good chance your child will latch onto that thought and want it even more, whereas if you go along, enjoy the party and not talk about it anymore your child will forget about it by the next day.