Is it a class for three year olds?
I wouldn't put a three year old in a class full of older children. She won't have their coordination or balance yet, and is likely to get discouraged -also, a lesson pitched at five year olds will be very different to one pitched at three year olds, no matter how keen or intelligent or gifted she is.
If it's aimed at her age group, and if SHE is the one who wants to do it (not you), then give it a try. Be prepared for her to lose interest quickly, though, even if she seems to love the first couple of weeks, so try to pick one where you don't have to pay a lot of money up front.