I have a 2007 Honda Fit which has gone through few limited configurations of 3-across. What saved me from having to buy a new car due to baby #3 was the Combi Coccoro whose width is about 15.5", and my two other carseats were able to be "puzzled" by having few inches overlap when installed. The Coccoro will not last long for extended rear-facing because a child will outgrow, but by then, you hope one of the other kids grows to move to a narrower or no carseat, thus allowing a larger/wider rear-facing carseat to replace the Coccoro for the youngest.
It's not the cheapest seat, but still cheaper than a new car. It's a convertible carseat, i.e. it stays installed in the car, instead of snapping it in and out like an infant carseat. (I never used an infant carseat because none of them had tethers or anti-rebound bars back then - a must for me, - so the Coccoro was fine.)
When every inch counts, the Combi Coccoro for the infant decreases the need of buying a bigger car. I hope you can find a workable configuration.