It is possible for a baby's head to change position within 3 days after a scan shows that it is down at week 36 of pregnancy. The position of the baby can fluctuate throughout the later stages of pregnancy, and it is not uncommon for them to move from one position to another. Factors such as the mother's movements, contractions, and the amount of amniotic fluid can all contribute to changes in fetal positioning. If you have concerns about your baby's position, it is best to consult with your healthcare provider who can provide guidance and monitor the situation closely. They may recommend additional scans or interventions if necessary.