We were in same boat 2 years ago. Likes of Verso, Scenic etc middle seat not big enough for three full car seats and don't forget that even if you can squeeze them in you have to be able to access seat belts etc (in due course even if they're all Isofix to begin with). So while they may look like they fit they have to have a little gap between them or else they won't restrain properly in an emergency.
Having looked at them all, Citroen Picasso etc, the only three that really fitted all 3 were the Ford S-Max and the Honda FRV and the Fiat Multipla each of which have 3 full seats in the second row. S Max has benefit of 3rd row when needed but the Honda FRV always has a full boot available at all timeswhich is necessary for buggies etc.
In the end, we chose the FRV as the S-Max was more pricey and we wanted Honda reliability (FIAT didn't really fit the bill there). We have been happy with it. Good to drive, not too long. It is quite wide though. The middle seat at the front has Isofix Points and has come in handy from time to time (transporting Granny etc). Most of the time though it's folded down and we benefit from very wide access to the 3 in the back. They also have a better view of the road as a result.
We read lots of opinions of Passats, etc being able to fit 3 car seats but they can't. The choice is as above if you want all 3 in the second row otherwise one of them will have to go in the boot !
The Honda FRV is no longer being made new (2010 was last year) but there are a few '05 models on Carzone. It's well worth a look, you might get the '06 or '07 (facelift model) for your budget.
Sorry, of course should have clarified, that the S-Max, FRV and Multipla are the SMALLEST cars that will fit 3 across, the larger Ford Galaxy, Sharan, Alhambra, Espace etc will fit them but I sense you're not into them.
By the way, I think far too many people are obsessed by getting an '08 car for the lower tax and Diesel etc. The Diesel premium makes sense only if you're going to do a lot of mileage. The tax payable does grate, but don't forget your paying a premium in the asking price for the saving you're going to make in future years, so I think people often overpay now to get this saving in future !
Any questions, feel free to ask.