IMHO, I don't think you're going to get as precise an answer as you may desire. There are too many variables, especially the height of the G3 Pros over the operating water level, if you have them hanging length-wise to one another or side-by-side as others do, and what distance apart you plan to hang them. I, too, tried to figure out what to do when planning my 150G reef (54"Lx30"Wx24"D) and did a bunch of research and questioning, receiving similar responses from EcoTech and others across various forums and inquiries last Fall.
Right or wrong, I moved forward towards the end of 2013 and locked in my cabinetry design and build, therefore my lighting design (long story). Based on the same "36x36" response I received from EcoTech for wide angle TIRs, I purchased 2 G3 Pros (not 3 as others may). I replaced the lenses with the wide angle versions, and built my entire cabinet and hood around that assumption using them in a length-wise manner -- purposely slightly closer together in the center of the tank with less coverage at the ends given my return is on one end where I don't need as much PAR, the other end where I don't want additional light spill, and the center should therefore have a bit more overlapping coverage and increased intensity than it otherwise would. My Radions are just a tad over 9" over the top of the operating water level of my tank and exceed EcoTech's minimum height recommendation (and thus far, splashing and similar problems do not seem to be an issue on the lenses or fan housing, and I check them daily during these early startup days.) I'm sure some will tell me I don't have enough light with only 2 G3 Pros, but then again, I am OK running two LEDs at 100% if it comes to that vs only 50-70% if I had an extra Radion as so many seem to. What practical results I end up with months from now, and what combination of corals I both desire and thrive in this environment, who knows?
My tank is just one week into its cycle. Initial corals and clowns are in separate QTs. Measuring PAR at various points on my DT aquascape is on my to-do list perhaps in the next week or so. I already know I'll want to measure not only at various depths, but also how far away from the approx center of the lights (front-to-back) I am when I do it... all the key locations where corals may be placed in my aquascape one day. My empty tank looks just great with the two Radions from a coverage/shadow perspective through the 10, 12, 14, 18, 20K spectrums my Apex is controlling and I'm playing with now, never exceeding 60% intensity as a starting point. Visually, the wide angle lenses accomplished what I wanted in terms of getting the spread out to the 30" width I have with only minimal light-spill onto the floor beyond. The front of a rock projection 3" from the front glass face still look good at night where I'll likely have a Zoa colony. Given my aquascape is higher in the back with a clear channel behind it for flow, the black back glass wall and the Powerheads on it disappear fairly well as I wanted them to throughout the day -- but the higher peaks in the scape always look almost snow-capped in terms of intensity compared to the rest, so I'm hopeful those areas will provide the highest intensity areas I'm looking for, and possibilities for demanding inhabitants one day.
Now, the $64K question is, with my wide angle TIRs, what is PAR at the points I care most about in my tank now that it's got water and nearly cleared up? I don't know. If I come up with something I think may be useful to others, and IMO is conclusive enough to fend-off the negative posters and haters that seem to live to poke at others, I'll put it up on RC in the next couple of weeks when I get it organized.
Good luck. I consider these lighting design points of what it "may provide" to be almost as bad as crap shoot in Vegas. At least with a tank, once you make a move with lighting, as time progresses, you know where you are and can create a POSITIVE way forward, not keep gambling and hoping your way into the future!
