A great game to calm down excited kids is "Toter Fisch" (dead fish). Basically all the kids have to do is lie on the floor and not move. You make a big deal of it, walking around saying "ooooh is little so 'n so moving", "aaaaah Susie is not moving an inch", "hey look there's an elephant" etc etc. Every time one of them moves, he's out of the game, and obviously the winner is the one who is last to move.
I don't know if three year olds would be into this game yet, but I used to play this with my friend's son and his little guests at birthday parties. They actually used to ask to play it, and it really did calm them down too.
You could also buy a big roll of paper (they sell rolls at
IKEA), roll it out on the floor and let the children draw a big picture together.
How about stick the tail on the donkey (or nose on the clown, whatever..). You could draw a donkey or clown and cut out tails or noses for each child to try and stick on to the correct position, while blindfolded of course. Kids like a good giggle, and when they see that the tail ends up on the donkeys ears or eyes etc, they'll think that's great.
A friend of mine always serves carrot sticks, sliced peppers and cocktail tomatoes, as well as the usual cakes and sweets. I was surprised that the children did actually eat any healthy food at all. Of course they went beserkers on the sweets first, but that's all part of a kids party isn't it.
Have a great time!