My sister has three clear almost perfectly aligned black spots on her heel. she say's they don't hurt. it's not marker. what could it be?

See a doctor: See your doctor and have them examine the spots to make sure it is nothing serious.

See a doctor: The spots would need to be checked by a doctor to find out what they are.

Sorry, "spots' is not a diagnostic description. Your sister should see her GP or a specialist Dermatologist or a specialist Podiatrist for an examination in order to be ab le to arrive at a correct diagnosis. Only with a knowledge base can correct information for going forward be offered.