I have an '06 Equinox. I'm not sure how three Marathon's would fit. If you did it RF, FF, RF, it might work, but it would be tight. The problem I find is the armrests on the doors. The Marathon might sit up high enough to not interfere with the armrests on the doors and give you more room. I do not have a Marathon, so I can't say how it fits with the armrests on the doors.
As Lisa (azgirl71) said, we did do 2 FF Radians and an infant seat, SS1, as a three across and that was pretty tight. All three had to be seatbelt installs so I could get the Radians as far outboard as I could to get room for a seat in the center. That doesn't help you much though since you need 2 RF and 1 FF.
As for how the Radian works in the Equinox, it's worked great for us. Even though it is tall RF, since the rear seat of the Equinox moves forward and back, I just move the rear seat all the way back to get the Radian RF outboard, giving the front passenger or driver enough leg room (DH is 6'2", I'm 5'8"). It also works well FF, I found that I get the best install with the buckle stalk twisted at least once, sometimes twice, and the recline foot of the Radian down.
Do you have somewhere you could try seats out to work out a combination? I'm curious about how it will work, I think I will have to get together with Lisa (azgirl71) to see. Unfortunately, our schedules aren't clicking to be able to do it soon.