Is Zakat obligatory on the amount of 20,0 Egyptian Pounds that was kept for three years, knowing that it was later withdrawn and spent on school fees and medical treatment?

It is obligatory to pay Zakat on money designated for needs for every year that passes while it reaches the نصاب (minimum threshold). If you have not paid it, it is obligatory to pay it now for each year, according to the amount of money at the end of each lunar year. If you are unable to pay it immediately, then pay it when you are able.