As per Vishnu Puran, three yugas namely Satya, Treta and Dwapara yuga have witnessed the avatars of Vishnu ji.
The Matsya Avatar is known for helping King Satyavrata from the massive flood.
He balanced the Mount Mandra on his back for proper churning of the ocean between gods and demons.
Varaha avatar or boar incarnation of Lord Vishnu defeated the demon Hiranyaksha and protected the earth.
Narsimha is the final avatar in Satya Yuga, and He killed Hiranyakasipu and established dharma.