We had to go to a minivan. We tried every car at the time and could not fit three into a car. There may be cars now that are able to fit three. As far as "skinnier" car seats, it seems as if they have gotten bigger! What we did was first measure our car seats and give them a little bit of room to buckle, and see what the dimension was. Then we took a look at new car seats. That's when we found out that they were all basically the same. Then we tried almost every car out there with our current car seats. That's when we bought a used Honda Odyssey. I swore I would never drive a minivan. Well, I do love it now, and it's been great with holding the stroller and all of the things that the older kids need. We now have four children and we are really glad that we have the extra space. I couldn't imagine having a smaller car at this point, especially as the older ones are now into sports which means bringing not only a stroller for the youngest member, but also a sun umbrella, chairs, a small cooler for food and drinks for everyone, changes of clothes....... the list goes on and on.
Good luck on your new car seat or car search. And congratulations on your new addition!