As others have stated MPVs tend to be the best with three child seats as they have individual seats each with isofix mountains. I have three boys aged between 5 and 10 so have had this problem for a while. We’ve had a Citroen C4 Picasso and currently a VW Touran. The Touran is a seven seater with isofix on the two rear seats as well. A Few of the others mentioned are also seven seaters which gives even more flexibility to the seating arrangement.
Current Car
KIA Sportage GT Line Mhev
Previous Cars
Peugeot 5008 Allure Premium
VW Touran SEL
Citreon C4 Picasso