Tried it this weekend. Fit a small adult 5 foot female, between 2 full size front facing car seats. Definitely not ideal. Can do it if you have to squeeze in 7 (or 5 and need cargo space). However, 2 adults in 3rd row with 2nd row all the way forward (car seats in that row) was not bad at all.
How old are the kids? Maybe the car seats can go in the third row? Are they forward-facing?
I have put three car seats in just about anything: Ford Contour (tightest fit), Chrysler PT Cruiser, Mazda Protege5. Couldn't have fit three identical large seats. And the space between two of them would have been tight for an adult, but doable. The JX is much wider, so technically there's enough room. But sitting between two seats will be like sitting in a hole. If you get two different seats that can overlap their bolsters, put one in the center and one on the outside, then might they both fit on the 60 portion of the second row? The adult could then sit on the right.