I can get two Radians and a backless Harmony in my Ody third row. I have an older model (the pre-2005 body style), so I'm not sure if it is the same as yours. That would be my first suggestion - put the two Radians in the third row on the side where there are two female buckles so they are buckled next to each other. Then put your backless on the other side (may need a new backless, but the Harmony (Literider or Cruz) or the new Cosco Top Side at Target are good options. If you have a newer van, there may be a LATCH position which will help this out if your DD2 or DS are under 40 lbs.
Mama to three boys ('03, '05, '07)