Wow nice diagram! What program did you use to render that?
I recommend checking out an article I wrote on connecting and calibrationg multi subwoofers:
http://www.audioholics.com/tweaks/get-good-bass/subwoofer-connection-guide
Don't rely on the readout of the SMS-1 as it 1/3 octave smooths everything. You really need a good 1/24th octave or better RTA to get a clear picture of what is going on in your room.
It seems placing 2 subs along opposite midwalls is not an option in your room, so I think you're doing pretty well with the near diagonal placement you currently have. You may want to try moving the back sub to the opposite wall where the first sub is located just to experiment.
If you're gonna do 3 subs, I'd definately try placing 2 of them under your mains like a T2 system and the 3rd one on the back corner where you currently have your second sub.
You should run all the subs in mono via the sub out/lfe port of your receiver.
Good luck and let us know how it goes.