wow, surprised on how unsupportive these answers are. Sure, can't make the care any bigger than it is but...
- one tip would be to set the front seats to the highest position possible. Obviously, it has to be comfortable and safe for the driver, but perhaps they are ok with a little higher. They can try. This will leave more room for the rear passengers to put their feet. Everybody thinks of pushing the front seat forward, especially of the passenger, but not many people think of putting the front seats higher. You can adjust the steering column to adjust for a higher driving position as a driver as well. Obviously, the less high your feet are, the more they can slide under the front seats, so rear seat passengers could even try taking their shoes off, lol
- I doubt you have a super early model 3. But the v1 seats (up to vin 25k) 2nd row bottom cushions are super thin and prove even less thigh support. V2 2nd row bottom cushions are thicker and provide more support. Tesla can swap them out for $500 + labor or you can attempt to do it yourself with a used car part. Downside is that v2 seats don't fold as flat, but it's still ok in my opinion. See this video
- There is a bit of a difference between bottom cushions of the 2nd row in the mY vs m3. mY's middle 'seat' is wider. I don't think it feels different and I don't know if you can put a mY bottom rear seat cushion in a m3, but you could look into it. Main difference between a m3 and my rear seats is that mY bottom cushion is about 15" of the ground vs m3 12" See this helpful video
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- that last video also says that the m3 2nd row is more comfortable than the mY. I just came from the Tesla showroom and tested them back to back and did not perceive any difference in the bottom cushion. The back support is uncomfortable in both. Even a tad worse in the Y in my opinion but don't know why. If you are willing to invest some $$, you could perhaps find a way to make that back support for the middle seat softer (bottom of the mid arm rest). I don't know if it is possible at all. I wouldn't know who or where to ask but I would go to a custom car shop (that makes hotrods and the like) and see if that is a little project they could tack on. It might be ridiculously expense for what it is, but on the other hand, another, upgrading to a bigger car will be way more expensive. The hotrod shop might even have ideas to make the bottom cushion more comfortable (e.g. make the front part even higher?). There might be after market mods for the rear seat, idk, would obviously be way cheaper
This reminds me of when we went skiing in our Prius II with 2 adults, 3 kids + car seats + 5 sets of skis, chains, food, clothes,... I bought a XXL Thule box and bars and a specific set of luggage to maximize usage. That extra gear was easily $1500 but avoided a $5-$10k car upgrade, so it was a no brainer in my mind.