Congratulations on your upcoming Baby #3!
I don't know how a 3 across situation would work in the S40. I checked car-seat.org's
3 across thread and didn't see an entry for the S40.
Rather than 3 Radians, you might find that you have better luck with something like a Graco My Ride or two to puzzle over the Radian. A My Ride + Radian facing the same direction takes up less space than 2 Radians next to each other, because the My Ride is able to overlap the Radian.
Another narrow seat to consider is the Combi Coccoro. It will not last as long rear-facing and forward-facing as other convertibles, but it is very narrow.
The Baby Trend infant seat is narrow, I believe. I don't know much about it, including whether or not it is difficult to install with a seatbelt. In order to install 3 seats, you'll likely need to do all seatbelt installs, because you can sort of move the seats closer to the door while installing to buy a little more room with seatbelt installs over LATCH installs.
When the baby is born, the 19 month old should still be rear-facing, and ideally the 31 month old will still be rear-facing as well.
This blog post has links to many resources to explain why the new recommended minimum for forward-facing is at least 2 years old, with the National Highway Traffic Safety Administration recommending until 4 years old as long as the child fits in his or her convertible.
The My Ride should last most kids until 3-4 years old rear-facing, and the Radian will last longer. The Radian has an angle adjuster that can be purchased separately so more upright install for older children can be attained. The Classic Model Britax seats (available at Target) should puzzle with a Radian as well, but they have lower rear-facing weight limits than the My Ride (35 versus 40 pounds). I'm not sure if the newer Britax seats puzzle with the Radians, and while they have 40 pound rear-facing weight limits, they have shorter shells than the My Ride, so they won't last as long rear-facing.
My own kids were able to rear-face depending on our car seats in each vehicle until the 3.5 to 4 year old range.