Top tether may not be a must, but it is STRONGLY recommended. Pilots have tethers in all six rear seating locations, which is awesome.
I have no idea how much the body style may have changed, but in my 2006 Pilot I think it would be likely that you could fit three Radians. You would have to have a FF and a RF Radian on the 60 portion of the bench, and that might even be a tight fit. Having a RF Radian back there requires all of the tricks to get it as upright as possible, and having the second row, which must be slid all of the way up.
I know there have been body style changes, though, so I'm not sure how that many have changed.
I would think buckling the FFers would be difficult, though. Truthfully, if any of your other children *could* RF in a Radian, I would RF them in the third row and load through the hatch... it's a breeze to buckle back there, I do it daily
My current combo is in my signature; I can also sub a Monterey for the Coccoro, and also put the Coccoro in the place of the MyRide. (So RN/MR/Cco, RN/MR/Mont, RN/Cco/Mont.) Be aware though that it's very tight back there.