How can hair loss occurring in a three-week period in a 2-3 cm diameter area be cured?

Alopecia: It sounds like you have alopecia areata and the short answer is there is no cure for this. Stress is usually the cause. Get rid of super stressful situations if you want it to stop, but remember that sometimes even that does not help. Sometimes steroid shots into the area of hair loss may help to encourage hair regrowth. You may also have a fungal infection, you may be balding, etc.