My Model S P85DL USAA policy costs no more proportionally than my Prius policy, so I don't see why the Model 3 should carry a big premium. This is especially true because much of the 3's bodywork is steel, not aluminum.
How do you know that? People at the delivery event took magnets to the car and said the bodywork is all aluminum. So if there is steel it is in the frame not the body. (Which is probably how they got it so light.)