I know around where I live it has become way out of hand with families having a catered meal. The “birthday” boy and his friends are really not interested! Are you looking for an activity for 3 year olds or for mixed ages? Keep it short, as young children have a short concentration span. Balloon games, or singing games are ideas I would suggest.
cupcake decorating – give each kid a small bowl with a bit of cream and a spoon, some sprinkles and other edibles to put on top.
or go to oriental trading and order a simple arts n crafts – do a search for ‘color your own’ on their website and you’ll get alot of inexpensive and great crafts!