A.P.
Sunshine Kids is infamous for making after-market products (ie: products that should never be used in or around carseats) but they make FANTASTIC carseats & boosters!
Their seats are all steel-reinforced & are some of the safest, most well-made seats on the market. And as you said, they are very narrow & will fit 3 accross in even small cars. I have several friends who have their seats & their kids all find them very comfortable. (I have at least 2 friends who have their school-aged kids harnessed in their Radians & they love them!)
We have the Monterey booster to use as a "back up" seat for our daughter (she's 6 but still in a 5 pt full-time) & it's fantastic! Most of my CPST friends use the Monterey for their kids b/c it's one of the safest out there & will grow w/ your child.
Just FYI, you may be able to find some good deals on www.HipMonkey.com if you use the coupon code "20Thanks"--it'll give you 20% off & all of their procedes bennefit the Kyle David Miller Foundation.
Hope that helps!