When you say one could go in a booster, you do mean a harnessed booster, correct? Because based on the tickers your oldest is 3, so at the very least it's a year before she can use a booster, most likely longer depending on her size and maturity.
For a 3 across situation you're best having everyone harnessed if possible. Getting hands in between seats to buckle a booster is a pain to do every day (in fact, when #3 arrives #1 will be moving her booster to the third row of our car because I don't want to deal with that on a day-to-day basis). Getting 3 seats in small cars is almost always possible, but you may need 1-2 radians to make it happen. For your older child you might want to look into the evenflo securekids 300/400 as that is the narrowest seat that has reasonable top harness height and also becomes a booster later. Also, instead of installing seats using the LATCH system install them with the seatbelt. The LATCH system pushes seats in towards the center a bit, so using the seatbelt will often buy you a few inches, which may be crucial in this case.
One other thought if there's absolutely no way to get 3 in (which I'm pretty sure there is). Do you need to be able to transport three kids in both of your cars? We will be unable to fit all three kids in our second car because it's a 4 seater, not 5 (it's a beetle) and we know whoever has 0-1 kids with them will take the beetle and whoever has 2-3 kids will take our bigger car. Until this summer our only car was the beetle and we had to buy a second car anyway, so we can't replace that. The only way I could see *needing* both cars to accommodate all of your kids is if one of you does school drop off and the other does pick up and there's no way to coordinate it such that you trade cars during the day. Otherwise you can probably sort things out to deal with only one car that fits the whole family.