what i see alot at little kids Halloween parties, is Winnie the Pooh.
all the characters are dressed up as Ghosts, and Tigers, and other things.
Im sure you will find Winnie the Pooh decorations, and it wont be to scary for the kids. also you can find little pumpkins at a grocery store for like a dollar each, and the kids can draw funny faces on them and take them home.
also, kids would love to play something like a scavenger hunt for candy, if candy is included, they'll most likely love it.
i would say bobbing for apples, but there 3 year olds, so that may not be a good idea.
and for the food, check out this link;
http://www.google.com/search?source=ig&hl=en&rlz=1G1GGLQ_ENUS267&q=food+for+kids+halloween+party%3F
hope i helped, and good luck!