You stated it seems to mostly happen during the afternoon, perhaps she's not sleeping well during nap, and becomes tired and more emotional and frustrated. I know some of my children react in an emotional way when they are tired. Have the teachers/providers mentioned anything about not napping well? Just a suggestion. Also, she could possibly be anticipating her pick up, I know some children in my care act differently when their parents arrive, and they tend to get a little more harder to keep entertained, which is why we have more structure during the morning. Also, I've heard numerous times during trainings, that children can become stressed when transitions are taking place. Anything changing in the home or center? You mentioned her switching age groups soon. Or that she is older, perhaps she feels as though she's not getting the attention she's craving because the younger children in the center are in constant demand of the provider's attentions. Another possibility could be changes in their children, has a child left or joined recently? A new teacher/provider hired, or has another recently quit? Children also can become stressed, emotional and flustered when they experience too much daily structure and stimulation or when there isn't enough. And of course, lets not forget that children, especially three years of age, are constantly and always testing and exploring adults boundries. They want to see how far they can 'push the limits' or how much the specific teacher will allow. These are all avenues that should be explored. I'd bring these questions up at the conference with the teachers and providers. I bet you can pin point the cause and help her transition or deal with what is bothering her. Children are 'little adults' they can become stressed just as much as we adults can.
I hope the situation reveals itself and of course resolves.
P.s. I also wanted to note that as a childcare provider, it is my job to help console a child and always try my best to help determine the factor in the situation and help rectify it WITH the parents help and cooperation. First of all, I think it's great that you are willing to help and get involved when they show that the situation is preventing them from providing adequate care for your child and the other children at the center. Also, I noticed some persons disagreeing with the center approaching you and asking for you to intervene, and I think it's great that they feel as though they can do this. I've never sent a child home with behavioral issues, (illnesses, of course) but if I ever got to the point where I felt I couldn't continue caring for other children because a child wasn't cooperating I'd like to know that I have the parents full support. GREAT job Mom!!!!