In my last house, I had 2 subs integrated very well after many, many REW sweeps and sub placement, settings, etc. Flat down to 20Hz before any sort of roll-off. Then I got a god deal on another sub and I thought adding it would/ could(?) make the system better.
After much experimentation and testing, the third sub did nothing at all for that system. I only had a few areas I could place another sub and none of them offered any sort of advantage.
So I guess the answer is, it depends. As usual around here.