I would try one and see how you like it! You can always build more.
I mean, it would be more efficient to make three at a time, but it will still take you much longer to build three than to build one.
You have two T30s, so lets compare the T30 to the THT so you know. Ooops, can't find a T30 loaded with a delta12lf. I'll use the T30 with 2x2010. That's gotta be close.
It looks like the THT is at minimum 12 db above, which is basically a lot.

I think that must be at least 10 T30s, but I think you get some gain if you V plate them. Also, you probably can put more power into your T30s. So the output from 4-10 T30s, perhaps... The THT also goes down a lot farther which you will certainly feel. 50 hz may be that chest kick, but 20 hz feels smoother and vibrates the floor. You will like it - a LOT.
Why aren't you building it yet? Cutting a hole in the floor will only slow you down from building your first THT!