Potty training at 3 horribly wrong. Help.

Julie

So my daughter will be 3 in April. She has been using her potty sporadically off and on SINCE 18 MONTHS OLD. However, she refuses to get rid of diapers. She knows exactly how this all works. Have read many books,had many successful potty times, gets tiny treats for using potty, has even had dry diapers overnight and gets up to go straight to potty. However, I told her today diapers were going bye bye and we were going to have a potty party and wear our new panties she picked out. As soon as we started she was really excited, but within an hour and a half she peed through 7 pairs of underwear even as she was standing right next to her potty and laughed each time. I've explained she can do more fun activities such as going to school,taking swimming lessons,etc when she is done with diapers and she really wants to do them but just wont. She is extremely bright and even professionally evaluated (long story)and was cognitively way ahead of others her age so she knows what she is doing. She even tells me when she needs her diaper changed and asks for clean diaper. Any advice? She has to be potty trained by August or she cant attend preschool.