I'm currently expecting dc3. We have a 2001 mondeo and the car seats that we have currently are graco boosters, having done research online there aren't any narrower one out there. There is only 13" left in the middle & with baby seats being 17" wide it simply doesn't work.
However googling 3 seats in the back of the mondeo brings up quite a lot of forums. Each forum has one or two posters saying that they easily fit 3 seats in the back of their mondeo (same shape as ours). So what are we doing wrong? I ca't see how it works, except for putting one of the older ones in the boot!