is 3HP too much? this is a small trough and I think it is pulling too much water too fast. Might try closing the return a little so water stays high in the pump strainer basket. The pump keeps losing prime. I have the system off now overnight to see if there is a leak. If not then will adjust the return valve and see if that helps keep prime.
But I still think 3HP is too much just to pull water from the trough into the pool.