My son went about 2 months before he turned 3. There wasn't a screening or anything. He went two days a week for 3 hours each time. He was the youngest in the class at the time but he adapted well. It was good for him to learn about rules and getting along with others. Our public school has 4 year old kindergarten which he now attends every day for 3 hours. I am so glad that he had the preschool experience first. On the first day, there were many kids that were crying and scared because this was thier first school experience. My son had no idea what they were so upset about. I had to explain it to him later.
I would try it with her for a while. She will probably really start to enjoy it. If you think she still needs more time you can always take her out and try again later.
Good luck.
C.