New only? We had the old style Picasso bought precisely on the basis of having three full sized adult seats in the back to accommodate three car seats.
Got rid of that when we had the fourth (!), but fortunately the first had outgrown car seats by then.
Ended up with a Fiat Multipla which also fits the “three at the back” bill, and avoided having to put anyone in the boot area of a cheap-ish MPV like a Zafira.
Fortunately the Multipla has lasted 8 years now (only one left in a car seat now) as they don’t make ’em any more. Nice wide roof too, so this summer on the way to the Alps we had three bikes on the roof, plus a roofbox, and four bikes on a towball mounted carrier, plus 6 people inside. Suspension travel wasn’t great…
With current regulations (and to be honest, best practice) with regards to child seat safety laws in the UK, you’re well advised to think through the car choice when #3 is on the way. Baby seats fit fine, but the older seats for 2-8 year olds can get quite wide so plenty of cars can’t fit them across the back. I don’t know about you, but I’d rather have a car that can carry my offspring safely than compromise their safety by forcing you to go down the “booster seat” route earlier than necessary.