I was hoping that our Model 3 would get delivered before our 2nd child, but she arrived in April 2018 and I picked up the Model 3 in late June 2018.
Our older child is 4 now and just switched from rear facing to front in a Maxi Cosi Pria 70, which is not a small car seat. I am 6'0" and had the rear facing seat behind me on the drivers side and did have to adjust the seat forward just a little bit from where I normally would have it, however, with front facing I have plenty of room to have the drivers seat where I like it. (Also, yes you can use the top anchor.)
On the passenger side we had our younger child in an infant seat (which I feel like any infant seat, even with a base, would be fine) and then later when she got bigger had the same Maxi Cosi Pria 70, and we have done 3 multi day road trips this way with no problems.
I have also had just one of the car seats in and folded down the other seat and middle to haul stuff (including 8' ft lumber). I have had the one car seat on the drivers side, the infant seat in the middle seat and an adult in the passenger side back seat. And I have had the car seat on one side a teenager in the middle and an adult on the other side of the back seat for over an hour to go to a baseball game. Yes, it is a bit tight in the last 2 scenarios, but its not too uncomfortable and works a lot better than taking 2 cars when you are going a long way and have to pay $20 per car for parking when you get there.
We also very frequently put a BOB Revolution Flex jogging stroller in the trunk with the wheels on when we dont have much more to carry or take the back wheels off (which is very quick and easy) to fit it sideways in the very back of the trunk right up next to the rear seats and there is tons of room left for other stuff, even access to the subtrunk.
The inside of the back seat and the trunk are definitely bigger and easier to access than our Volvo S40 which was out daily driver before.