Car seats for 3 rear facing children

I am currently pregnant with twins and my daughter will be about 25 months when they are born. We have a Yukon XL with captain chairs in the middle row which I didn't think twice about having an issue with car seats until now. The car seats that I have (one nuna aire rx infant car seat and one nuna exec next convertible) do not fit in the 3rd row at all.

For parents with 3 in rear facing carseats what did you do? What car seats did you use? I want the car seat with the best crash testing and would prefer flame retardant free. Would it be easier to just get a bench seat for the middle row? What configuration did you use in your car? Any advice is appreciated!

Edited to add: I would prefer the twins be in and infant car seat to begin with because then I can just carry them to and from the house in their car seats or in the stroller but if convertible is the best option I will have to be ok with that too. I really don't want to buy a new car.