Note that the Base S does not have leather seats. They are an option.
I am sure they will also be an option for the 3. How "similar" the 3 will look compared to the S is of course a subjective judgement. I for one will bet that the 3 front end will look very different. Rear may well be similar, and side profile will likely be similar except for a higher rear deck to give the second row passengers more headroom. That is necessary because of the shorter wheelbase and the fact that the motor/inverter is just behind the rear seats.
You were spot on.
Nice!