Re: Is it possible to fit 2 marathons+1radian
At those heights/weights, I'd go ahead and leave the 3.5yo ffing in the Marathon. How much room does he have to grow? (How many inches b4 his shoulders reach the top slot?)
You could leave your 23mo in the rfing radian and put the new baby in a new rfing Marathon 70 (will prob need to buy the infant support wedge to make it fit a newbie),
or you could put new baby in the xtsl (it came w/ the infant padding to make it fit an average newbie) and put the 23mo in the Marathon 70 until she gets to 40 lbs (then switch the 2 so your middle child can rf longer).
Those combos should fit across a Jetta Wagon b/c you'll have both a rfing Radian and a rfing Britax. They puzzle really well together and should leave plenty of room for almost any ffing seat (including the classic marathon).
Make sure you install all 3 with seatbelts (it'll give you a lot more room) and check to be sure all seats are independently tight.
For the far future - when it's time to turn your middle child ffing, you'll have to turn the Radian. That will keep the radian facing the same direction as one of the Britax seats and leave you plenty of room for the 3rd seat.
S, mama to my Second Grader, K (9/07) and my Kindergartener, e (7/09) and married to my best friend, E (2007)