I have a similar setup to Andywrx. RF radian behind the driver (fits without the angle adjuster if you're not too tall), passenger captain stowed, booster in the back latched into passenger side. If I added a third seat for some reason, I'd do it next to the booster in the back. Having the one captain stowed makes it so much easier to get kids in.
Soontobe- would that setup work for you too? Then there's no one in front of the RF in the back row. I haven't tried the radian in the third row with a seat up. Just from eyeballing it, I think it might fit since Diono allows you to brace, as does the pacifica (correct me if I'm wrong) since there aren't any sensors in the second row seats.