I joined the Imam in the last Rakah in Asr Salah. So when I now pray the remaining 3 rakah do I sit for tashahhud in my first Rakah that I pray on my own or in the second rakah that I pray on my own?

Ustadh AbdulAziz al-Haqqan

The 1st rak'ah because you have already prayed 2 rak'ah with them meaning this is your second rak'ah thus you make the tashahud.