Bud Seal U md-i and 3 seats

No, max 2 child seats. Technically you can fit 3 booster seats, but middle will cover seatbelt bit for both middle and right space.

Yes, I have 7, 5, 1.5, as of now I am using 2 full child seat for older ones and bucket/carry cot type one for the infant. It fits perfectly. By the way, I’m NOT using the ISO fix. Very comfortable and not squeezed. We have done a 10 days trip from Austria to Southern Italy and all were very comfortable.