Yeah, what kind of car do you currently have?
When we got pregnant with #3, we knew we didn't have enough room to legally put three carseats in the back of a Subaru Impreza (two was snug enough as it was!).
We got a new Honda CRV. I had 3 Britaxes in the backseat. Lots of carseat back there, but it worked (the oldest was in the Frontier in the middle, second on the side in a FF Marathon, littlest in her Snugride then RF Roundabout).
Then we got pregnant with #4. Not planned, not expected, nothing. Oof. We ended up selling our CRV and getting an older Honda Odyssey. I'm not the minivan kind of gal, yet here I am.
My choice was basically a giant SUV for all the kids (just because they're younger doesn't mean they can do without any leg room in my book) that only got 12-15mpg if I was lucky, or sucking it up and getting a minivan that gets closer to 20+mpg. Some people kept telling me to get a Mazda 6 or something, but they weren't going to be buckling kids in the back over the other rear-facing seats, nor were they going to be available to haul hundreds of pounds of apples or strollers or whatnot for me at my whim.

I haul stuff and kids on a fairly regular basis, I need a decent cargo-like area regardless of what I drive.
It
is nice having the extra seat now, though. Grandma can go with us to the fireworks, or the kids and I can take a mommy friend and child with us somewhere sometimes. I'll get my 1960's Landcruiser in 10-15 years, I'll deal with this in the meantime.

Oh, I
do have three Sunshine Radian 80SL's. Would 95% recommend them. The base is 15" wide, shoulder of the seat is where it's widest at 17". I have three, forward facing, across my narrowish older bench seat in the van. Not sure how much room they'd need rear-facing, but they are certainly nice and narrow. Odds are they'd fit nicely in my mom's Camry if needed. Hubby's older Prizm would be a challenge, but still.
Another tidbit I've learned - the more kids in carseats, the more head room you want. Otherwise you're going to have a perma-bruise on your forehead from whacking into the door frame or roof of the car.