Two subs is the max configuration for Sonos.

EDIT: But, thinking  about using an Amp for the front speakers, you MAY be able to add a non-sonos sub to the sub out on the Amp in addition to the dual Sonos Sub setup.

Someone correct me if my thinking is wrong?

  Bestbuy is correct. Click the link.

https://support.sonos.com/en-us/article/set-up-your-sonos-sub?utm_source=www-care&utm_medium=sonos-sub-4%2Fcompatible-products&utm_campaign=en-us-www-care-sonos-sub-4%2Fcompatible-products

BTW….why would you need 3 subs

Thanks is there anything I can do for more bass?

Make the physics work for you. Bass is highly dependent on listener and speaker placement.

Walk the room very close to the walls carrying some Post-its and a magic marker while playing a very thumpy track. You will notice a spot where the bass is accentuated. Mark this spot with a Post-it. You will also notice a spot where the bass seems doubly accentuated. Mark this Post-it with ‘!’. Continue around the room. As you step back you’ll notice that the Post-its are more or less symmetrically placed around the room. These locations are known as “Peaks”. About midway between peaks there are “Nulls” where bass is minimized. For maximum bass, place the listener and SUB at Peaks.

In my college apartment I discovered a monster peak and placed a chair on the peak. This was a favorite spot for males because bass exploded from within the body. If I moved the chair a few inches the bass explosion was gone.

Multiple Subs are tricky to place, you can end up with less sound than with just one.

Try the suggestion above but follow-up by plugging your Subs into extension cords and actually trying them in multiple locations.

If I wanted the most boom I'd look to corner placements, set at 45 degrees to maximize room coupling. Nulls might be an issue so try multiple corners. That got one Sub to work well here in an audio-awful L shaped open plan room.