We have a Ford Galaxy (aka Seat Alhambra, aka VW Sharon). It has three rows of seats, which is how we fit our three kids in - there's a seat (folded down usually) between the twins in the middle row. That seat is big enough for an adult to sit in (with a little discomfort) but it isn't really big enough for a full "young child" seat. It may be okay for a "cushion booster" seat, but it would be quite tight.

To be honest, I wouldn't want a car any wider than the Galaxy - I think going to three rows will give you a lot more comfortable journeys than trying to squeeze everyone into one row. We're very pleased with our Galaxy; in particular, our back row only has one seat in so there's a lot of room for luggage, which is very important when you're taking three children on holiday :)

Many full size cars (US EPA class "Large Cars") will fit three narrow car seats, so they should be fine with two car seats and one without. However, due to fuel economy changes, even some large cars have reduced width over the last few models, so it's not as true as it used to be, and even if someone says it fits in a specific car you need to ask the year of the model as well, and perhaps the country they purchased it in. Further, a "full size car" in the US is larger than what others might consider "full size" in other countries, so your best bet is to take two car seats and a child down to the dealership and see what fits.

We recently purchased a Mazda 5 - which is smaller than a mini-van, larger than a full size car, and has three rows. The middle row seats the younger kids in boosters just fine, while the bigger kids can go to the back seat and buckle themselves in just fine.

It's priced similarly to a full sized car, and drives like one, so it may be an option for you. It also has dual sliding doors for the back, which you may or may not prefer, but it is really nice for helping kids get in and out and buckling them in cramped parking spaces - also don't have to worry about the kids opening the door too far and hitting adjacent cars.

We were very pleasantly surprised by it, and are getting great gas mileage to boot.

It doesn't replace our windstar minivan, but works well as a commuter car for me, while still fitting one adult and all the kids so neither of us is ever stranded at home with the kids regardless of which car the other took.

It will fit 4 full sized car seats in the back two rows, and has automatic airbag sensor to turn off the front airbag if you do have to sit a child in the front seat.

Keep in mind that car seats have different widths. It may be cheaper to buy new cars seats that aren't as wide as the seats you currently have than to buy the wider car, if you can find a midsize, or small full size car that fits the narrower seats.

We have a Subaru Legacy wagon, and my husband can sit in the back between our two full sized car seats. He is not a large man, but he would surely be bigger than your eight year old. I don't think it would be very comfortable for long trips, but it is possible.

We have a Honda Civic and will be able to fit our 3 kids by using 2 Radian car seats (the narrowest on the market) and a backless booster. Kids are 7, 4 & will have the baby next week. It'll be a tight fit but will have to do for 2 years until we can upgrade to a minivan.

We bought a Toyota Prius recently (see here). While we only have one small child, and use only one carseat, I encourage you to try installing two seats in the back of a Prius and find out if it is wide enough. I suspect it is.

EDIT: to answer the comment below: I'll grant you it's not a car for drag racing, but I wouldn't want one of these for my family anyway. Regarding the environmental cost of the battery, this is true for all rechargeable batteries. All batteries can and must be recycled! If the battery is recycled properly at the end of its life cycle, the fuel efficiency far outweighs the environmental cost of battery manufacture.