I've been dealing with a bad lower back for about 35 years (surgery at age 19). I have to be pretty picky about seats and I find the Model 3 to be quite comfortable. It took a little while to find the sweet spot, though.
The Gen 1 Model S seats were not great for me. After about 1.5-2 hours, I was squirming. Not so, with the Model 3. I guess I have the Gen 2 seats in the 3 - July build, black.
One thing to remember about the Model 3 is that you can move the steering wheel much more freely. Since there is no instrument cluster to peer at through the wheel, you are free to adjust it purely for comfort (and usability, of course). A different steering wheel position can allow different seat arrangements. Don't forget to try all the options.
For reference, I'm 6 feet tall, 185 pounds and I'm one of the long-torso builds (my torso thinks I'm 6'4" but my legs know better). I find it best to have knees and elbows bent to about 90-120 angles, which is consistent with doctors' advice. I keep the seat fairly low for head room, pretty far forward for my short legs, and tilted back (IOW, I raise the front part of the seat for thigh support, which also tilts the pelvis back). I also keep the back reclined a bit more than most people find comfortable -- not gangsta-low, but pretty low. My wheel is positioned so that I can see the forward part of the dash over the top and telescoped towards me, to keep my elbows comfortably bent.
I know that's not really likely to work for many others, but it works well for me. The point is to use all the variability you've been given. A 12-way seat is 6 degrees of freedom, plus the wheel is 2 more. That's a lot of options to play with. Good luck!