I suspect that the RF Marathon will fit fine. It might fit better in the center than outboard until the baby is ready to sit more upright. Just try it out and see before you decide.
Assuming everything fits, I'd put RF baby outboard behind the passenger, FF 3 year old in the center and FF 6 year old outboard behind the driver. Switch 3 & 6 year olds if the 6 year old is needed to help keep the baby happy (giving her a pacifier, etc). Otherwise, just go with what fits! Radians are so narrow, I'd be surprised if you couldn't just put all 3 kids in the second row.
When you try the Marathon outboard, push the passenger seat all the way forward, install the Marathon at the correct angle tethered, then slide the passenger seat as far back as it will go (don't run over the tether!). Sit in the passenger seat, and if there is at least 10" between you and the front dash, you're good to go. As the baby gets more mature and can sit up better, you can put her seat more and more upright.
Here's a picture of my daughter Eva rear facing in a Marathon in our old, TINY Saturn SW2. She was very upright at that point, but the seat did fit with room for me in the front. In fact, I could push the front seat almost as far back as it would go without the Marathon behind it! I installed the seat just like I described above. If you put the MA in the center, do the same thing, only push both the front seats forward before you start to install.