If you exchange the Blvd for another Radian, you can do three across the back seat of pretty much any 3rd row.
This is my '03 Grand Caravan with 3 Radian's across the back row...
This is with a Marathon, Radian, Parkway...
As for third row, I would set up the vehicle as follows...
Van with bench...
RFing Radian, FFing Radian, SafeSeat1
walkway, Regent, Regent
Passenger, Driver
Van with captain's chairs...
Safeseat1, RFing Radian, FFing Radian
Regent, walkway, Regent
Passenger, Driver
In either case, it will be a little tricky to ge the SS1 into the 3rd row (through the lift gate might be the best way), but it needs to be that way because of the rear facing Radian. The Radian RFing in the 3rd row needs to have an open space in front of it because it extends slightly too tall into the seat in front of it... that's why the seats are set up like that in the third row.
Once the baby is out of the SafeSeat, everyone can move up a seat and your then 10 year old'ish can go into a booster.
Vehicles...not sure if they came with Captain's chairs or not, but either way you should be able to fit two Regents on the 2nd row bench if you got that option. These model years all came with the option (or pre-installed) tether anchors in *all* 3 positions in the third row. You'll need that once the kids are FFing.
1996 - 2001 Dodge Caravan/Grand Caravan
2001 Plymouth Voyageur
1999 - 2003 Ford Windstar
I have no idea what their prices are but that should give you somewhere to start. Hope it helps.
One more thing... you'll want to read this thread for install tips when putting the Radian in forward facing.
http://www.car-seat.org/showthread.php?t=28939