I haven't tried it - no more car seats for us - but here's one thing to think about: it's enough of a pain to load a kid into the car seat in the middle position. It's even more of a pain when the roofline is cut low. My ten-year-old hit her head when getting into the 6 for the first time. Granted, she's used to jumping into an Outback, but I'm not sure I'd want to deal with that on a daily basis if this is your main car to carry the three kids.
Belt-positioning boosters aren't that big, so I'm betting you'd find two of them plus a rear facing seat will fit. Another issue is that the buckles are kind of recessed into the seat and if a car seat is sitting in the middle, I wonder how easy it would be to dig out the buckles for the older kids. The same ten-year-old complains about having to dig out the buckle.