Looking at the data from my multisub experiment, the first sub didn't add much to the response, and the fourth and fifth left the response looking nearly the same.
Compared to the nearfield, fast bass subs, the response looked pretty good:
The sound difference matched that graph more than any of the other Time Response views.
So with my three subs, I placed the subs where the second, third, and right in between the fourth and fifth placement. The response came out similar:
Black lines are with 3 subs placed to approximate the 5 multisubs placement.
The lack of deep bass is due to the cheaper subs actually having a deeper response.