The secondary regulator is meant for the lower psi beer(s), so first of all I wonder if the gas line through the secondary regulator feeds a "y" splitter or manifold serving your 2 lower psi kegs, as it should. Then, assuming you have another gas line via a splitter off of your co2 tank regulator, it would go to the Belgian beer and be set at the higher psi.
If you are trying to reduce the psi through your secondary regulator, turn the secondary regulator screw or knob counter-clockwise until it drops down below your co2 regulator pressure, to the desired lower pressure.
You would first have to bleed off excess pressure thru the secondary to read lower pressure after adjusting the screw/knob.
I hope this makes sense to you.