My daughter just turned three but she has a hard time mixing with other kids. I don’t know what to do.

3 years old is still quite young to learn to socialize well. Is she attending any playgroup or preschool already? If not yet, then it is quite normal for them find it hard to mix with other kids. You can join playdates with neighbors and slowly start from there. Otherwise can sign her up for some short hours playgroup. If she is already in school and facing hard time to mix with friends, you can talk to the teacher and request the teacher to prompt her in class to play with others. Slowly she will have friends and more comfortable to mix around. Here are some tips to encourage your child to mingle with others: http://www.theparentszone.com/parenting-tips/parenting-tips-to-help-your-child-mingle-with-others-and-make-friends/