Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
You need to make one rakah, which is the third for you. In the third rakah, you need to recite Al-Fatihah only
Answering your question, Dr. Muhammad Salama, PhD, Islamic Studies in English, Assistant Professor at the Faculty of Islamic Sciences al-Madinah International University (Mediu), states:
According to the most preponderant juristic opinion, for a Muslim who joins the imam in Prayer after missing some rakahs (prayer units), the number of rakahs begins with the first rakah he catches with the imam.
So, when the imam finishes the congregational Prayer, those who missed some rakahs stand up to complete what they missed.
Hence, for the case in question, you need to make one rakah, which is the third for you. In the third rakah, you need to recite Al-Fatihah only.
Allah Almighty knows best.