I run something simular. One overflow is a full siphon that's adjusted to the return pump, one is open all the way and vented to drain the the excess with the vent hole routed with tubing just above the overflow waterline to create a full siphon if the first one fails or water gets to high, and the last one is just below the top of the tank line (full siphon) for a triple back up if both overflows fail.

The only time I hear water is when my filter sock gets clogged and runs over in the sump.



Top view when I was building it.