[sameh]

If you turn off the car, on ANY gear it'll go to park "P" abd thats how you'll find it when u turn it on. Is this enough or not? It depends:
Some people in manual engage a gear an leave the car like that.
Some do the above and add ebrake.
Some automatic people just keep it on P while others also pull the brake.
So it's the same really for an M3, do whatever you've been doing all your life. Personally, I put it on N, pull e brake then turn it off.