The ISOFIX requirement is the big constraining factor. Ditch that and you can find combinations of compact car and child seats which will fit three across, though you’ll have less choice in terms of child seats.
Even if you ditch the ISOFIX requirement, it's still difficult to find a combination of 3 car seats that will fit across the back bench, even with large cars.
ISOFIX points can be found in the outer rear seats and in the front passenger seat.
So then you most likely wont have space for the second parent.