I stopped by my local Tesla showroom, which received a three-row Model Y last weekend. I wanted to see how a rear-facing car seat fits in the vehicle. The upshot is that I was not able to fit a rear-facing car seat in either the second row or third row without fully pushing back / forward the non-car seat row.
A few additional observations:
It is recommended that kids use rear-facing car seats until they are at least two years old. Depending on the car seat and weight of the child, that age can go as high as five. For those who have kids in that age range, you might not get too much use out of a third row.