I have noticed a trend in 3 year olds that after months of coming happy every day they start to go through separation anxiety big time again and there can be severe moodiness at daycare, tears at drop off, agressiveness towards the parent at pick up, etc. It is actually considered "normal" behavioiur and has to do with the child's increasing awareness of their independence. After spending so much time with parents and then suddenly back to separate places all day can set them off for sure. If you google separation and three year olds you will probably be able to find some articles to share with the family. Be careful to treat the child matter of factly and not put too much emphasis on her feelings or she may think there really is something to be upset about - just go about a normal day in a normal way but mention the child my name more often or ask her to do something for you, serve a favourite food for snack, do an activity she likes but in a way that she doesn't realize you are doing it for her.