part two below
What I am saying is that the outside key will open the drivers door mechanically, (not via the push button), and then the retrieved key will start the 3. The outside key will not start the 3, (only open the door as the start allow electronics are not within the dealer supplied free key fob). I note that you can purchase empty key fobs on eBay. The reason for the key fob is just as a handle for the metal key blank. Anything that retains the blank would be OK, however the LR key fob makes the "key" look like it will do the whole job when it will not even operate the remote, let alone start the 3.
This concept is pretty simple but it requires that the mechanical door lock work. I say this as I exercise my mechanical door lock about twice a year just to make certain it has not frozen from lack of use.