Thanks for the info. S-Max does seem to be a great option for 3-under-4. I also have the same issue (will have newborn, toddler and a 3yr old by next spring) and so have also done a lot of research into this, and it appears MPVs are the only option.
I'd wanted an estate, but even the Mondeo and the big Volvo estate won't fit three proper car seats in them. I'm sure if you compromise your childrens' safety and use boosters without a headrest etc then you might just about squeeze them in, but why on earth would you take a chance with your child's life?
We haven't bought our new car yet but it will be S-Max, Citroen Grand Picasso or Renault Grand Scenic, all of which fit three big seats across the back.
While I'm here having a rant, I also have no idea why car companies that market their cars at families don't bother to put detailed info in about precisely which car seats will fit. I have spent literally hours online trying to find out quite basic info. Even Ford s-max website doesn't clearly explain what seats fit where.
Oh, another thing to be careful of is that most MPVs have under-floor storage (WHY WHY WHY???) which means you can't transfer any seat that uses a leg, as it's unsafe. So we will have to buy three new car seats for which ever one we go for.
The whole car seat thing is a nightmare and it's no wonder most people go round with unsafe or incorrectly fitted seats as it is so difficult to find out the correct info.