Watching this as my brother has exactly the same scenario, and the same problem. The problem we seem to be having is two fold, firstly the underfloor comartments, secondly having a car wide enough to get three baby seats in (more specifically two baby seats and a toddler seat). To date we've ditched the isofix idea and are looking for Ford S-Max.
If you go for a ford with storage compartments you apparently can buy fillers to make them safe but they seem to be the only ones that do