A.P.
I had my 3rd almost 2 years ago and did exactly the same thing you're doing. My oldest was 3-1/2 when the 3rd came. We have a Dodge Magnum and got the Sunshine Radian for the older 2 and used the Snugride for the baby. My research showed the Snugride to be the skinniest infant seat.
When it was time to move her into a convertible seat, we discovered this Combi seat (referred to me by the fire department). They say that 3 can fit in the backseat of a Toyota Corolla: http://www.combi-intl.com/Products/Item.aspx?Item=12
When it was time to face the baby forward, the 2 Radians and the Combi wouldn't fit so we ended up buying a Chevy Traverse (I too am anti-minivan) with 3 rows, however we could probably have gotten away with replacing one of the Radians with another Combi until the oldest was in a booster.
Good luck. Three so close in age is pretty tricky, but it's so much fun to have 3 best friends playing together.