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Well, you have to potty train her - in a different way. You indicate she has been potty trained since 19 months. That's quite young, so likely what happened is you got her body used to going on the potty at specific, probably pre-defined and regular interval and it always included YOU taking her or her telling YOU she needed to go.
But potty training is about more than just being able to go potty on the potty. Seems weird, I know..... but it's actually mental development as well as physical development and then throw in maturity and verbal skills.
What you need to do is potty train (again, differently) her so that SHE is in control of feeling the sensation and taking herself to the potty or verbalizing to whomever is present that she needs to go to the potty. Maybe start going places outside of the house where she HAS to use her verbal skills to ASK to go to the potty - instead of at home where she is comfy and knows the routine.
This sounds like a maturity issue, which isn't bad, but may indicate that she either isn't ready for pre-school or just needs some additional time to get used to it. However, peeing 3x in 4 hours seems excessive. Almost like she is doing it on purpose.
I agree that you should not put her back in diapers. Instead just send extra clothing to school each day. Let her know that until she can go in the potty at school she will be soiling her clothing and will have to stop playing in order to clean up and change. This is the very definition of natural consequences.
Good Luck