R.,
I just went through this same thing!!! My son turned 3 December 9th and it sounds like his stage started a little earlier than your daughters. It was horrible. I got to the point that I would cry after dropping him off at school because of seeing him cry! He was being very mean to my husband - didn't want anything to do with him (which broke my husband's heart and started making him angry after a while). He only wanted me - and in the morning getting ready for school he didn't want me to leave him so that I could get ready. I was late to work almost every day for about a month.
I have a 5 year old as well and I tried to get him to help me by leading by example. It didn't seem to work very well. One day prio to Christmas he snapped back into normal. I don't know what it was, but he just became the nicest boy and decided he wasn't going to cry at school any more and that he was going to be nice to his daddy.
Now, just before this "snap back to reality" I started being pretty harsh with him. If he was mean to his father (i.e., not giving hugs or kisses goodnight, or telling him to get away) I would not do what he wanted me to. For example, if he didn't give kisses goodnight to Dad then he just got taken straight to bed and laid down instead of our normal routine (book, rubbing back, & prayers). In the morning when I dropped off at school, I would walk straight in, sign him in, put his stuff up, and give him a kiss and walk out (all very quickly).
Now, I don't know if any of these things had anything to do with the "snap back," but he did come back to normal. Now I know it was a stage and possibly a result of the age. My oldest went through separation anxioty at this same age, but didn't get mean and clingy - just didn't want me to leave him at school. I did talk to my youngest about school to see if something was bothering him, but the stories I got were very contradictory (sp), so I never felt like there was really a problem. Talk to your daughter though to see if you can pinpoint something and then confirm what you are hearing with the teachers. Ask the teachers how she is acting at school after she settles down from the morning drop off.
It will pass, but I hesitate in saying that it probably wont be quick and it will be painful. I hope that for your sake she snaps out of it quicker than mine did, but know that regardless she will snap out of it. Keep to your routine as best you can. Try not to give in to everything, but pick your battles. Remember that you are the parent and while you may cry about it in the shower over how you treated her 30 minutes ago, she will be better off for it. It is okay to leave them crying in their room and step away from them for a minute - go outside so you don't hear the screaming if you have to (I had to do it a few times and cried along with him).
Good luck R. and know that you are not the only one that had dealt with this. Try to find comfort within your daughter when she is being kind and tell her how happy you are that she is being so good.
Sam