I notice black spot in heel 3 weeks ago?

Could be: Many things from a wart to skin cancer. Get it checked specialy if its changed shape, raised or color. Don't mean to alarm you but better safe than sorry.

ABC's: Although i agree with all the posts previous, use the abc's (for melanoma) to help relieve some stress before you see a doctor is the spot asymmetrical? Are borders irregular shaped? Is the color uniform or multicolor? What is the diameter? Also is it elevated? Lastly, of course get it check-ed out but those abc's should help relief some stress. Hope this helps!