Which fast is Sunnah: fasting Mondays and Thursdays, or fasting every 13th, 14th and 15th of every month?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Both are Sunnah.  So, any of them you performed fits into Sunnah.  But you could combine both; that is better.

Allah knows best.