I personally would not like a rack on top of my M3 unless it were easily removable because it would always steal some efficiency and range from the car as well as impact the aesthetics negatively. Plus there is almost always another better solution than semi permanent roof bars.
For
bikes and such a hitch system (which I believe is available for the M3 from aftermarket companies) is a better solution.
For
kayaks, SUPs, surfboards etc you can either do the sea sucker thing or something simple like this:
https://www.amazon.com/b/ref=dp_bc_6?ie=UTF8&node=10208178011
For
skis and snowboards I do not know what people are doing for the M3, but I think sea sucker has something for that too. I personally put them in a bag and slot them down the center of my 6 seat MX. I wonder if there is space to do this in the m3 if you fold down the 40 part of the 60/40 folding seats? I keep requesting Tesla to put a little passage door one can open in the middle of the M3 seats like they have done on some VW's and other cars like this:
View attachment 356987
