I currently have a Honda CRV. After much swearing and deliberation, we managed (well, the lovely people at InCar Safety Centre in Milton Keynes did!) to fit 3 car seats - 2x high backed boosters and 1x infant seat and base.
We managed this by using:
Britax Adventure (I think) - the smallest footprint hbb available, I believe. Fitted on passenger side, for dd1 age 9. She also needs an extra suppport harness fitted, due to challenging behaviour.
A Two Way Elite fitted rear facing in the middle for dd2, age 7, and about to outgrow this height wise.
A Maxi Cosi something (pebble?) plus base for ds, now 17 months old. HE has a bit of room left, but as we are about to change cars, I need to make sure that his next seat up will fit as well, iyswim?
So, I need a car which will fit:
2x hbb for the girls, and an ERF seat for ds - could be the TWE, or could be anythign else which will fit, tbh. We do have an Izikid stored away, but this is quite a monster of a seat.
There are a few extra bits:
dd1 still need the extra harness, and cannot do up her own seatbelt, so I need to be able to reach the fastenings.
ideally, dd1 would not be placed next to ds (although we may have to bite the bullet on this as if dd2 is in the middle, then I have to unload dd1 as well at drop off since I drop dd2 off first).
Can't put ds in the middle, as don't want to be lifting him over to strap him in (he is a chunk!)
I want a 4x4 again (dd1's school is in ridicuously rural location). We have a Discovery already (dh's car) - while this fits 3 seats across the back, it is almost impossible to do up the seatbelts between the hbbs, leaving me standing in the rain fiddling about for ages!
We can't really have (well, can have, but can't use effectively) a 7 seater, as can't sensibly use the rear row of seats - can't put dd2 there, as I drop her off first, so would need to get dd1 out (and her seat) or put dd1 on the middle seat and then usually the car's seat will not fold down past the hbb, and can't put dd1 there as she can't do her own seatbelt, and I can't reach to do it, plus the same car seat folding down issue.
Nightmare!
Any thoughts?
(and thank you if you have read even half of that unintelligible rant 
)