My DH has a Mazda Premacy and we can get a Britax Nordic Secura, a Nania baby bucket thingie (don't know the name of the model) and a Britax 2-Way Elite all rear facing. Not yet had 3 DCs in the seats since I'm still PG with no. 3 but I had to try it out before we started trying to conceive just in case we couldn't do it and needed a new car for 3 DCs.
Our back seat is three adult sized independant seats with 3-point seat belts so if a car seat doesn't go over the edge of the seat we can get 3 in.
I phoned up these people and I got great advice. We're going there over the summer to get the fit checked as well as them looking at the Nania seat as it is a real cheapy to see if there is a better model on the market now that also fits.