hello fellow scion driver!
It might not be as hard as some might think because the center seating position has a significant raise which with the right seats will make a 3 across set up much easier.
you MIGHT need to get a more narrow seat for your 3 year old, if you're still using the myride. maybe something like a diono convertible seat or even the graco mysize 65/size4me 65 will free up a couple more inches of width.
for the newborn are you wanting a rear face only seat/infant seat or a convertible?
narrow seat options are the chicco keyfit 30, graco snugride 30, cybex aton. all of these seats are also mindful of the limited front to back space available too.
do you live near a babies r us? you can try out the graco seats and chicco key fit in your car to get an idea.
you shouldnt have any problems accommodating your booster rider.
hope this helps!!!
if it doesnt come back and let me know and I can help brainstorm some more!