Help me choose a third carseat (three across)

Hello,
This site is so great and so informative! Right now I am expecting our third child (due in June) and looking for a few options regarding fitting three across the backseat of our 2003 Durango.

children are:
DD - 6 years, 43 lbs,
DS - 2.5 years, 37 lbs about 38" tall
Child to be - probably under 7 lbs given history. DS was a preemie and I would like to have a seat that works for a preemie as well as a term baby.

Currently we have a Radian (65 I think? Whichever was available in Canada in 2008) and a Marathon. My original plan was to take DD out of the Radian and put her into a booster seat. The seatbelt configuration in the back is shoulder-lap, lap, shoulder-lap; so she would have to be outboard. Then, taking the Radian rear-facing, put the new baby into that. However, now that we have purchased the Durango and have kids in it and driven it for awhile we are realizing that there is not as much space between the front seats and back as in our previous vehicle. I'm not wanting to be kissing the dash the whole time I'm in the car because the Radian takes up so much room rear-facing.

The other option is to buy an infant car seat for the new baby. It would still have to fit across the back bench with three across. I was thinking Britax because it has a 4lb weight, but it seems so wide - any other suggestions?

I would try the Keyfit. It also has a 4 lb minimum weight, and is great for itty babies. I think if you have the Radian in the center and the Marathon on one side and the Keyfit on the other, it may work. The Keyfit is one of the narrower infant seats, and the base is narrow too.

Oh, and boosters are not usually very easy in narrow spaces, think about how you would have to squeeze your hand between the seats to buckle it every time!

I too don't really like a booster in a 3 across - but I do it and scrape my hand up a little more each time

But I have to say Radian rear facing in the middle with two FF seats on either side does puzzle really well in a 3 across. It was my 3 across for a year and if I had the money - it would be again! But I have no idea how a radian would fit a NB 7 lbs babe...