M.K.
i think at three you might want to consider a bike w/ training wheels- they make 12", 14" and 16" which are suitable for her age (depending on height). we decided to do that for our son, who will be 2 1/2 this month ( he is very tall tho, and pretty coordinated) he was already interested in bikes and we felt it was a better long term investment. also, he is around kids with bikes at the playground and already wants to be like the big kids. The first day we took him up there he got on the bike and rode after them as fast as he could calling "wait for me guys!"
we got a 14" and put the seat almost all the way down and the training wheels only 1/4" off the ground. he had no trouble riding it the first day.
One important thing to note- he was able to learn on a trike at daycare, so if she isnt comfortable on a trike already, it would make sense to start with the trike and get comfortable.
Enjoy your summer with the new trike/bike!