All perfect praise be to Allah, the Lord of the Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Blood that comes out of womb-after delivery or miscarriage- is called Nifas {the post-natal bleeding period}. According to our school of thought, Shafie,the minimum of Nifas is a moment and the maximum is sixty days. Therefore, a woman with such condition must stop praying and sleeping with husband during Nifas so long as it doesn`t exceed sixty days.
After the sixty days, the bleeding is considered Istihadah {Womb bleeding between periods}. If the blood stopped for three days, but continued before the end of sixty days from delivery, then it is Nifas and the period in which the bleeding had stopped {Tuhr} is Nifas as well.
Therefore, if you have fasted these three days of Ramadan, then you must make up for them later, because they turned out to be within the Nifas period, and so your fasting isn`t valid.
An-Nawawi {May Allah have mercy on him} said: "If post-natal bleeding stopped before sixty days from delivery and the Tuhr period {Blood stoppage} didn`t reach fifteen days, then it is Nifas blood and the fifteen days are Nifas as well." {Al-Majmou`}. And Allah Knows Best.