I have found in the past with CC that alot of parents tend to invite all the kids in the room and I would quite often turn one down as my kids didn't mention or interact with the kids.
That set aside...yes 3 is probably young for a Macca's party. An afternoon evening party would be a struggle for most 3yr olds I reckon as in overtired or just not in a party mood, it would be with my kids.
It's also possible that this child has older siblings who know of Macca's and he or she has had an early introduction to Maccas due to them having had it.
My DS1 went to a Macca's party when he was 3. It didn't end up with him asking for it afterwards, I just did not 'brand' the party so he really didn't think anything of where or what he is eating.

Macca's have come along way in food. It's probably better than a party pie, little boys and chips and sweets party that may be held at home or play centre that would also end up with a lolly bag.
Most of the kids parties will be a Happy Meal that would consist of either a cheeseburger or nuggets or wrap, choice of either fries or fruit, water or juice and toy.
The cake would more than likely be a Macca's icecream cake. I don't think they have any games as such, the kids would just play in the play area.
Hope that helps. Good luck with your decision.