sounds like adding the third sub created the condition.
as others have mentioned most likely too much energy in the room at frequencies shared by the main speaker.
1) first off- how do you have them connected? if two subs are connected to the same channel that might draw enough current to make the main speaker of that channel have reduced volume compared to the other.
2) what are the makes and model of your other two subs?
3) the SVS may also have cabinet resonance in the audible range that (unfortunately) the crossover frequency cannot control.
as others have mentioned most likely too much energy in the room at frequencies shared by the main speaker.
1) first off- how do you have them connected? if two subs are connected to the same channel that might draw enough current to make the main speaker of that channel have reduced volume compared to the other.
2) what are the makes and model of your other two subs?
3) the SVS may also have cabinet resonance in the audible range that (unfortunately) the crossover frequency cannot control.