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

It could be that she is shy. There's no need to force her, but bring her to more places where there are children around like indoor playgrounds, play dates, the zoo etc. It's best if she has cousins near her age to interact with on a frequent basis. http://www.parents.com/toddlers-preschoolers/development/social/helping-a-shy-child/