Yes, you can definitely get 3 across in an ION with radians I'm sure. The ION has 3 dedicated sets of LATCH anchors in the back and they can all be used at the same time, making it nice for installing carseats.
Are any of your kids over 48lbs? That's the LATCH limit in Saturns, and the Radian could be difficult to do with a seatbelt. I wasn't a tech yet when I tried it for myself so didn't try any tricks, but had real difficulty when trying to get it in ff'ing. Actually ruled it out since I thought a seatbelt install wouldn't work in it.
With the new AOE having 17.5" top slots and a 50lb weight limit, I'd be mighty tempted to get one of those. If you take the base off and use it ff'ing on the driver side, then radian center and baby bucket passenger outboard, you might be able to get it to work.
Here's a pic of a snugride/MA in our ION - the snugride was really being pushed slightly off center by the MA, and the MA was right against the door - couldn't be moved further outboard because the side of the shell was hitting the armrest.
So a narrower seat than the MA, and a slightly narrower infant seat and you'd have it pretty easy I'm sure. With a narrow enough infant seat you might be able to make the MA work, but I wouldn't hold my breath on that.
eta: Here's a link to a different angle of the 2 seats side by side. The way the MA pushed on the snugride wasn't quite as much as I remembered. The MA is pretty wide in the world of carseats, so a narrower infant seat and 2 carseats a bit narrower than the MA and you'd be set. If you get an AOE, just make sure it's one of the new 50lb ones since those have the high slots - actually higher than the MA, so if you don't need the extra weight limit, you'd be set. :thumbsup:
http://i5.photobucket.com/albums/y194/snowbird25ca/car seats/jan282007014.jpg