My suggestion for a fantastic 3rd birthday is to stay at home and have a party. They're still too small to really appreciate another venue and it will be less costly at home. Another fun idea if you don't want to stay at home is to go to the zoo and have a picnic but then you need sufficient adult supervision as little ones at this age can get lost with all the excitement of seeing all those animals.
Hope this helps, it's always difficult to plan a special birthday. I'm planning my little ones 2nd birthday and I'm doing at home. Looking for ideas on a doll theme.