I had my son at a wonderful preschool in FM, but they closed this past summer...Just in time for wanting to start my daughter there... =( But now I have her (she turned 3 in Dec and I started her in Jan) in the Early Connections program at the Lewisville Rec Center (for 3 year olds, it's called "Little School"). It's solely a preschool, and it's been great so far! My daughter loves going - the only thing I don't like is the time; it's only from 10:00-12:15pm Tues and Thurs. I wish it was a little longer, but next Fall she'll move up to the next class ("Busy Bears" I think) and it's for another hour or so, then the Pre-K program is a little longer than that.
I also know a lot of people who take their kids to Lewisville Christian School (Main St and Garden Ridge, right on the edge of FM) and they love it. For their 3-year old program, the kids must be potty trained, otherwise they put them in MDO, which, to me, is more like a daycare. But their preschool programs are just that, preschool, not daycare. Their times I think are Tues, Thurs, 9a-1p.
I also have a friend who just started her daughter at DKH in Highland Village - I think it's a little more pricey, but I have heard many, many wonderful things about it! Also just a preschool, not daycare. I have considered the possibility of switching my daughter there in the Fall.
You might also want to check out Reading Friends; we were going to send our son there, I really liked their program, but it was too expensive for us at the time. Good luck finding something, there really are a lot to choose from - just make sure you visit each place with your child at least once - they are usually pretty good about letting your child get in with the group and interact/participate with the class while you are there, so you can see how they do.