on Monday someone makes an intention that on Thursday he will keep a qadha fast (of Ramadhan). But he totally forgets that he has made this intention and on Thursday morning he wakes up and realises that he should have been fasting. Now since he has not, as of yet, done anything contrary to fasting can he continue to fast the qadha of Ramadan which he had initially intended to do?
One of the condition of a Qadhaa fast is Niyyat the night before. The earliest the Niyyat could be made is from after sunset, the night before the fast. In your case, the Niyyat was made 3 nights before the fast, so it will not be considered valid. (Aalamgiri vol.1 pg.197)
The condition of Qadhaa is Niyyat from the night.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai