Ok, so went to baby shop today and the following combination does work;
- Safe n Sound Compaq Deluxe
- Safe n Sound Compaq Deluxe
- Safe n Sound Highliner Booster

So although we may have to consider a change of car in the future, atleast for now we can get away with the 3 baby seats across the back. Actually quite suprised that they fit with not too much of a squeeze.