Question Three across in minivan

With baby #5 arriving soon, we need to figure out how to get 3 across in the back seat of our minivan.

We have a 2007 Toyota Sienna.

The seat we already have and don't want to replace that will go back there is a Britax Frontier (the original first one that came out that is now retired.). It is on the passenger side, forward facing, with a seat belt install. 4 year old DS sits in there.

Right now, my 8 year old DS also sits back there, driver's side, in a graco turbobooster with the high back. The middle position is empty.

My 2.5 year old DS is currently rear facing in an about-to-expire Britax Boulevard. Since that seat needs to be replaced, the thought was to switch him to forward facing, and stick him in the third row.

The fourth child is my 6 year old DD, and she is in a Graco Nautilus that we just bought last year. I KNOW we can't fit the Nautilus, the Frontier, and a booster in the third row, so putting her back there is not really an option. She is small for her age, and we would prefer to keep her in a harness as long as possible so I don't want to switch her to a booster, either, and her seat is new so I don't want to replace it.

So - about that third row.... if we get a Radian, which I know is the narrowest seat out there, what booster seat do you think would fit back there as well? It would then be a Frontier, a Radian in the middle, and then hopefully a booster seat. Preferably a high back booster seat.

We've tried already to stick a radian back there with our existing turbobooster, and my husband said it wouldn't fit. He didn't try removing the back of the booster, though, it's possible it may fit that way.

Thanks so much!

Liz

Hi and welcome. Congrats on #5

Eee. Tight fit, huh? That Frontier is going to be a challenge.
I have a Frontier 85 puzzled next to a RFing Graco Size4Me (aka MySize70). They puzzle perfectly. I think the original Frontier would work as well because the contours are similar and the armrests of the old Frontier are even lower.
The width of the Frontier 85 plus Size4Me is a 30" wide footprint and abut 35" wide at the height of the shell where the Size4Me flairs out. This is with the Frontier on the passenger side and RFing S4M in the center. Plus the shell of the S4M is high so that a booster could have room to buckle on the driver side.

I don't recall your third row having any strange contours in the seat that would make seats a challenge.
Would those measurements give you enough room?
At least with a convertible seat, you could keep the 2.5yo RFing a little longer (especially if the seat fits better RFing). I can try the S4M FFing to see how it fits next to a Frontier. I'll brb...

FFing S4M takes up even less space next to Frontier 85 than RFing by 1/4" at the base. I could fit them both on the 60- part of my 40/60 split bench. (2003 Lexus RX300) But the shoulder space of the two is wider than when the S4M is RFing.