We've had one for almost 2 years now and are still happy with it. I've only done 3 across once in it, but one of the reasons we specifically bought it was due to the wider back seat width than most similar cars. The doors also open wider than some, which makes access a little easier.
Link to the 3 across:
http://www.car-seat.org/showpost.php?p=1637899&postcount=8
We've also had 2 adult passengers in the back with the carseat, not infrequently, and never had a problem.
And just because I was curious, I had to look up the specs on the XTerra. The Soul's rear hip room is 6.5" wider! And supposedly 1.5" wider than the Equinox.
It's going to depend on the driver/passenger heights as to how well RF seats will do. We had an XTSL in the center RF for 2 years, and my 6'1" husband could put the seat all the way back, but it was a tight squeeze. He ended up putting a hole in the side of the front seat where it would hit the edge of the Radian every time he slammed the driver's seat back.
You would definitely need to try the RF Marathon behind the driver's seat and see if you can both still comfortably drive.