According to Madhhab:
My wife has one Qadhaa fast of Ramadaan left to keep. Can she keep that on the 9th Muharram, and the fast of the 10th the next day? Or does she need to keep 3?
Her fast on the 9th of Muharram would have been for the missed fast of Ramadaan which is an independent fast altogether. The fast of the 10th would be a Nafl for the day of Aashura and she would be required to join one more on the 11th due to the narration that states that one should oppose the Jews by fasting a day before the 10th or a day after the 10th of Muharram.
She would therefore be required to keep all three fasts in the queried scenario; the 9th being a Qadhaa fast which is a totally independent fast, whilst the 10th and 11th will be for Aashura.
AND ALLAH TA’ALA ALONE IN HIS INFINITE KNOWLEDGE KNOWS BEST
ANSWERED BY: Mufti Mohammed Desai
CHECKED AND APPROVED BY: Mufti Muhammed Saeed Motara Saheb D.B
Darul Ifta Azaadville