It is not a "bad" idea. However, bass is highly dependant on room acoustics. It's nearly impossible to say unless you've had 4 sub points in your room in the past vs just 3.
Chances are there wouldn't be a huge difference in response (if any), but no one knows for sure 100%
"If Bad Sound Were Fatal, Audio Would Be the Leading Cause of Death."
If you can do dual opposed in the back why no just build 4 cabinets? You can then stack two in the back to start and if you're getting nasty nulls you can then have more flexibility to relocate and move them around. Do you have a layout of the room?
I have attached a layout of the room. The subs pictures represent proposed / future sub locations. I currently only have one sub in the front right corner, which recently died.
I'm really after that deep chest pounding bass. I was initially going to only do two sealed 18" boxes. But after reading a bunch of the threads here (particularly Gorill83's thread), I'm wondering if two 18's will give me what I'm after.
how about one or two right behind that loveseat? that will help with you being on a slab.
two singles maybe? im adding two single 18's that will only be 12.75" deep. low profile to hug the wall under my screen. you could do the same thing for behind the loveseat if space is limited. firing into the back of the loveseat.