It's possible you could do a Radian, Harmony Defender (I wouldn't buy a Radian for a forward facing child), and the Chaperone baseless. But with the base I don't see any way of getting three across. Sorry! Even the Chaperone baseless would be a tight squeeze.
Don't go by just inches measurement. For instance, the Chaperone baseless might slide in and above two other seats, so the sum of their combined width is less than the sum of their measured widths.
The Radian for the 4.5 year old could be passed down later to the baby, then the 4.5 year old bought a combination seat or booster, depending on when that switch happens and how much room there is to buckle a booster. We generally don't recommend boosters until about six or older.
The Defender is just about as narrow as the Radian, but has higher top slots, a usable booster, then becomes a backless booster.
I've done in the past a Defender, forward facing Radian, and rear facing classic Britax Boulevard in a 2002 Honda Accord *sedan*, to give you an idea. I know the base won't work for the Chaperone, but if you tried it baseless it might just fit. If you could put the three year old in rear facing (under 40-45 pounds and 44") that would be safest as well as give you about three extra inches to play with. I don't think it'd be enough for the base, but it should ensure that baseless fits. For a secondary seat, I think baseless is a nice option.
Wendy