Wa-alaykum as-salamu wa-rahmatullah wa-barakatuh

In the Name of Allah, the All-Merciful, the Mercy-Giving

All praise is due to Allah. Peace and blessings be upon His Messenger Muhammad.

According to the most preponderant juristic opinion, for a Muslim who joins the imam in Prayer after missing some rak`ahs (prayer units), the number of rak`ahs begins with the first rak`ah he catches with the imam. So, when the imam finishes the congregational Prayer, those who missed some rak`ahs stand up to complete what they missed.

Hence, for the case in question, you need to make one rak`ah, which is the third for you. In the third rak`ah, you need to recite al-Fatihah only.

Almighty Allah knows best.