I no longer have my old style PWs, but I don't think they're any wider than the PWSGs. In fact, I think they may have been a smidge narrower, taking up less room. But because it sort of curves out towards the edge (where the legs hang), it can be difficult to get them side-by-side in a tight space. The reason you have no room in your 3rd row with a PW and MA is because, I assume, you're using the lower anchors to install the Marathon. Because the LATCH is off-center, when you use it, you totally lose the driver side outboard spot.
You could install the MA outboard with the seatbelt and have the PW in the other outboard spot and then probably still have the center open for a seatbelted occupant (not a large one, however).
Who would be going in the other PW?
ETA: Are your kids older than stated in your siggy? Putting 4-year olds in a booster wouldn't be my first choice, especially in a primary vehicle. The PWSG (old style Parkways aren't in production anymore) have a 40 lb weight minimum as well.