Serenity, I have a similar setup to yours. There are two choices:
1) You can connect all three subs to one sub channel on the 4520 and optimize the subs as you would on a XT system, not using the dual-sub capabilities of SubEQ HT. While this is a more complex approach, there are advocates of this approach when using three subs. I prefer method #2 below.
2) Utilize SubEQ HT as JD has already recommended. In my case, I followed these steps, which may be applicable for you as well. This procedure is meant for three identical subs--if you have different sub models, then the problem becomes even more complex, and approach #1 above may be better. It is also expected that you have experimented with various placements for the subs in your room so that you have a good idea which placement produces the smoothest response BEFORE running Audyssey.
- First, gain-match the subs as per the procedure here.
- Next, connect two subs using a Y-cable, plug the cable into Sub1 on the AVR, and locate the subs in the room so that they are equidistant from the MLP.
- Then place the third sub where you want it, and connect it to Sub2.
- Make sure the AVR is configured for two subs.
- Launch the Audyssey room correction routine and progress to the screen where the subs are level-set. Important: do not change the sub levels, regardless of what the Audyssey screen reads. You have already gain-matched the subs, and you don't want to disturb these settings. Audyssey will adjust the sub trims during the calibration.
- Run a full 8-point calibration.
- After it has completed, inspect the trims for Sub1 and Sub2. If either one is at the limit (i.e. -12 or +12), then you need to go back to the first step and run the gain-matching again, using a lower or higher gain setting. You do NOT want the sub trims to be maxed out after the calibration.
If, after completing this procedure, you feel the overall bass level is too low, you can use the sub trim adjustment in the AVR (do not adjust the sub gains) to achieve your preferred bass level.