When to make intention for fasting because of a promise?

Asalamualaykum, I have been currently making up my fast and I have finished my obligatory ones but I had 3 fasts which I promised Allah سبحانه وتعالى if He (the most merciful) made me achieve a certain something I would fast for 3 days. I am currently doing them and I didn't wake up for suhoor but made my intention at fair. I was told my fast doesn't count but I haven't eaten anything yet as I want to know if it would be invalid.And if the ruling would be the same for everyone or different.