* subs would be worth doing. I use 3 Submersives and that system works quite well. However, the OP is considering 2 identical subs, (dual F12's) and 1 sub with (likely), significantly less capability, (JBL 10"). In this situation, I expect the JBL sub to do nothing but hold back the F12's. At higher output, it will compress and distort long before the F12's, wasting their headroom.
More importantly, with less extension, it will cause the F12's calibrated levels to be lower, reducing the levels of their deepest output. For example, the F12 has extension to 14 Hz, (-3 dB). The JBL 10" sub is probably in the 28 to 35 Hz range. When the *combined* output of the 3 subs is calibrated to 75 dB with the speakers, the level of each individual sub is reduced 3 to 6 dB. The net effect is that the response below 28/35 Hz is reduced relative to the rest of the range.
The only situation I would suggest using the JBL sub is if the dual F12's have a large "hole" somewhere in the response. Then the JBL could be used to "fill in" that hole. However, doing so would require finding a placement that has a "complimentary" response to the F12's in the area of the hole. This would require the ability to measure the response, and the time and effort to find the correct placement. Short of that, I suggest leaving the 10" JBL sub out of the system.