Lets go back to first principles.
Besides smoother FR, more sub headroom and better amp efficiency, another major benefit from multiple subs comes from mutual coupling or "free" output. Depending on placement dual subs will generate up to 6db more output, 3 subs up to 9 db, and 4 subs up to 12 db. In Randy's case he appears to be getting the additional output of 9db. Here's how I would calibrate:
1. Make sure the sub trim at the processor is at -10 or well below the midpoint but leave yourself some room if necessary to lower sub trim further.
2. Make sure the speakers are level matched at 75db. Make sure all subs are bypassed and phase is at 0 . Measure the distance of the 3 subs plus 3 ft for the 600 DSP and another 3 ft for the SMS DSP. Divide by 3 and use that to set the distance for the subs.
3. Shut off the EP350v3s( let's call them A and B) and run a sweep to just the 600. Raise gain at the sub so that the 600 is calibrated at only 70db (that's right 70db). This may take time because the DSP circuit at that level is very sensitive to small changes in the volume control on the 600.
4. Shut off the 600 and 350v3 B. Run a sweep and adjust gain at sub 350v3 A to 70db.
5. Shut off the 600 and 350v3 A> Run a sweep and adjust gain at sub 350v B to 70db.
6. The only sub that should be on is 350v3 B. Now turn on 350v3 A and run the sweep so both 350s engaged. You should measure around 75db. If not check your polarity on one of the subs.
7. Turn on the 600 so all subs are on and run a sweep. you should measure around 76-78db.
8. Now using your SMS dispaly to see where to adjust the phase switch on the 600. Choose the phase which gets you the smoothest response not the loudest volume. The ep350v3s at the front should be at 0 phase.
9. Using the SMS connected to the 350s and running sweeps. Disable the SMS low pass filter and crossover. Adjust the variable phase in the SMS test menu in increments of 15 deg to find the smoothest FR.
10. Now use the SMS parametric filters to cut peaks only. You may need to go back and check the phase settings again once you have set the filters. With multiple subs the filters will sometimes raise peaks or lower nulls so experiment in both directions. Nice thing about all those subs is you aren't using up much headroom in the sub amp so you can raise the filters if necessary.
11. You should be close to 75-78db with all 3 subs engaged. You may want to check different crossover settings to get the smoothest response. Adjust sub trim at the processor to where you want your target SPL for all 3 subs combined.
12. Move the subs around slightly to find the best placement and FR for that configuration.
13. Change the delay in the processor while running sweeps to see if it smooths FR.
14. Go back to #8 above and review all settings one more time to fine tune.
Let me know.