The room wreaks havoc with LF response of any main speaker, that much you have proven (and it doesn't really matter who told you what about how low these open baffle main speakers are supposed to go; they cannot prognosticate that parameter exactly, just give you a general idea since room modes are a given). So the fact that the F3 isn't 60Hz is something I wouldn't be concerned with. Your F3, with the graphs you have shown appears to be 38 Hz. I'm drawing a line mentally at 85 dB to show you what I believe is the middle
. So for example, you are right at flat (0dB) at 70 Hz. You won't know what the 'middle' is unless you do some measurements extending above 300 Hz. Like up to 1khz.
Now play your subs, introducing one at a time. Play with position. Play with the crossover frequency, phase, etc.. of the ONE sub, and see how that changes the response of your system (2 mains + 1 sub). When it's as flat as possible, then fiddle with the other sub. Do the same thing. It takes time as you can already see.
See here: https://mehlau.net/audio/multisub_geddes/
Look particularly at the part that is titled "Exemplary data"; See how the response flattens below 100 Hz. Geddes prefers moving the mic in an ellipse while somebody else accesses the computer to tell REW to measure. But that isn't necessary. If you have an extra person to help, great, use them. If not, set the mic at the listening position on a boom mic stand and go for it.
The links to the HifiZine article in this post should hopefully help: http://www.audiocircle.com/index.php?topic=149872.msg1603064#msg1603064