Is it sufficient to make up the five days that I did not fast in Ramadan, believing them to be menstruation, then it became clear that it was istihada (non-menstrual vaginal bleeding), knowing that this happened three years ago?

A woman must fear God and distinguish between menstrual blood and abnormal vaginal bleeding (istihada), because each has its own rulings. What the questioner did is correct, and she must make up the prayers for the five days if she has not already prayed them.