There are quite a lot of cars with three equally sized seats across the back now. We had to get one when we had our third child. We've had a Ford S-Max and a Citroen C4 Picasso but there are many more.
I had a look on the car seat pages for you with 3 across the back and the admins on the page have said that it’s not possible. I would put the baby in the front with the airbag off as a rear facing seat in the front is safer than a forward facing seat. Although if you have an older child in a rear facing seat then I would put them in the front first. Hope you get sorted!🙂