Hi,
I think I'd pass the Scenera to the baby and get your 2y/o a new seat. Since you have a Radian you could always get another one. I'd use LATCH to install the Radians, and put them both outboard. I'd install the Scenera in the middle using the seatbelt.
We recently test drove a Corolla and Yaris (both '08's). The Yaris is skinny in the back, but the above combo might work. The Corolla you should have no problem. Take your seats and try before you buy.
Now keep in mind some people have difficulties installing Radians in Toyota's with the seatbelt. And you can only use LATCH in Toyota vehicles until 40lbs (though some techs have said it's okay to go to the industry standard of 48lbs in a Toyota -- it's a topic of debate, that's for sure), BUT, I wouldn't discount the Radian as an option w/o trying it first. You never know, but you might get a successful seatbelt install. Since your 3y/o is just 30lbs now, you would have plenty of time to install the seat with LATCH. Down the road, when you switch to a seatbelt install, if you encountered a major issue, you could always look at some other seat (who knows what will be out by then, kwim?). For now, providing you can get a good LATCH install, I'd go for it.
I hope that makes sense. :thumbsup:
I'm all about the 3 across set-up, and I'm *really* all about fuel efficient vehicles these days. What we're paying per litre is insanity. :twocents: