you doing it wrongly.
everyone thought you have 3 pipes for overflow, but the 3rd pipe furthest away is your return pipe am i right?
if this is so,
simple
with 2 overflow, do a herbie style. center pipe, full siphon without the air tube.
seal the air tube and adjust your valve till return pump is same as that full siphon pipe. this will be silence since it a full siphon without air
the other overflow pipe will be your emergency pipe in case of things stuck in the full siphon pipe.
so you just need to adjust till the middle pipe full siphon equal to your return speed and your emergency pipe is just there as emergency.
the emergency wont be doing anything.
if your valve is difficult to adjust to 100% full siphon with the returns , just adjust till it almost full siphon of the return and leave it, a bit of water flowing through your emergency wont make any sound.
cheers