I tried to explain in the private message but for the sake of information for all the audience we have here: 3mm is probably long enough for most of the people to cover up the scar signs in the donor area.
It also depends on how course your hair is. If it's thick, you shouldn't have any issues wearing 3mm. One other factor is the hair style. If your hair is cut to 3mm at the bottom (towards the neck) and hair height gradually increases towards the top, you should have no issue with scars what so ever.
To give you a better idea, please attach a photo of your donor area with the hair style you like. You can also provide somebody else's hair with the height you are thinking to have after transplantation.