I got my 3 about 3 weeks ago. I initially had some discomfort after an hour of driving. It took me an unusually long time -- over a week and 1000 miles -- to get the seat adjusted to what I like but, once I did, it is fine. I am 6'6" (+) so my position is different than most. The lumbar support in particular was hard to get right and more important than I expected.
So, my experience is stick with it and get it right but that might not work for you. BTW, my recent experience with 2018 Golf R and 2018 Volvo V60 is that it took me about 2 minutes to get the seat right. The 3 is apparently just a lot different.
I still dislike the fact that the seat depth (front to back dimension of the base) is less than other cars. Another inch would be much better but I like all the other stuff so much I live with it.
I also have driver profiles for: my normal position, more comfort (further back from the wheel on the freeway i.e. almost straight arms because the wheel doesn't come out enough and this lets me straighten my legs a bit more), me with somebody behind me (seat and seat back further forward than normal = a little cramped but still safe) and Easy Entry. I really like the profiles and Easy Entry, although I haven't used the comfort or back passenger profiles to speak of yet.