Firstly, I don't think you are starting to think about this too early! There is a lot of time involved to do the research you need to find the right preschool for child. However, I don't think you need to send her right when she turns three. Where I live at least, you wait until the September after your child turns three to start them at the first of the school year. If there was an opening somewhere, I suppose you could start her when she turns three - well, no, the schools I know require the kids to be 3 by August 31 of the school year. Your state may be different though.
I agree with Lauren that finding out which schools are knowledgable about NAEYC will be a good thing. Accredition is very difficult to get but I at least like the teacher to KNOW what NAEYC is! And hopefully go to the state conferences each year.
You do need to do a bit of thinking about the type of preschool you want. There are co-ops - where parents "work" anywhere from one day a week to one day a month at school. I personally think these are great for a child's first year of preschool as they go one day a week without you and one day a week with you - good transition for both child and mom!
There are Montessori preschools - usually more expensive and more days/hours per week than other schools.
There are religious preschools.
There are traditional preschools where you drop your child off but they are more like co-ops in the way they focus on learning through play.
There are private preschools - although most of these are either Montessori or religious.
There are daycares that try to pass themselves off as preschools. I always ask if they have day care or before/after school care. If they do, I cross them off my list.
You want to find something that is within a reasonable distance from your home, that has the number of days/hours per week that you want, that is in your price range. But the most important thing is the TEACHER! A great teacher can overcome a small facility or any other issue. Don't be impressed by the nicest toys or the biggest facility - focus on the teacher. She should obviously love what she does. She should have a calm control of the room - not needing to raise her voice or use much discipline to keep a happy classroom. She should have age appropriate materials and be very knowledgable of what is developmentally appropriate for this age.
I would start by making a list of basic questions-
days per week?
times of class? (morning, afternoon, all day)
cost per month?
student to teacher ratio and class size?
potty trained necessary?
teacher's focus? (some are academic, some are social)
I would take your yellow pages and call all the preschools in your area of the type (co-op, Montessori, religious, etc.) of preschool that you are interested in. Asking if you can ask a few quick questions - and those answers - will help you figure out which schools you want to visit. I would go alone (without your child) to the ones you are interested in then go again a second time to those you really like with your child to see where your child seems to fit in best/connect with the teacher best.
Also, asking friends for their recommendations is a good idea but not the end all be all. Some of my very good friends send their kids to schools I wouldn't be caught dead in. Everyone has different things they are looking for in a preschool.
Good luck finding one you and your child love!