I had the exact combination of seats that you are needing when no.3 was born. I don't have a Saab but a Landcruiser, despite their overall size their back seat is not very wide. I ended up at the In-car safety centre and spent ages with them going through combinations. I ended up having to get a Maxi-Cosi Robi high back booster as it was 2cm narrower than the Britax one we had. The small width difference meant the difference between them all fitting and not. I found that you really needed to get a place with experienced car seat staff and try all the seats in the car.