I mean, I haven't been in the back seat of a Model 3 personally, but literally every car I have ever had, analog handles or not, you had to do something to make the air flow to the back. Even if you have vents in the back, the air doesn't go there by default. This was usually a button on the dash - so I'm not surprised if even in a Model 3 you have to select an option on the screen to make the air re-route to the back. This is not a critique btw, just a possible observation.