For what it's worth, I would not tow much of anything with a BMW sedan. Before I got into Airstreams, I had a small Jayco pop-up and a 5-series BMW - the trailer was less than 1000 pounds, well within the tow rating. I got the approved hitch and had it professionally installed, but the problem is that there is no frame to speak of in the rear of the car. It's uni-body construction and the frame rails are relatively thin and made of stamped steel.
Within a few months, the hitch started tearing thru the frame rails and started pulling the bumper off the car. I had plates made and welded to the frame to reinforce the attachment points, but was never comfortable after that and got rid of the car shortly thereafter.
I can't speak about your 3-series, but I would look closely at how your intended hitch would attach.