Not many of them are aware of the fact that Lord Vishnu had chopped Lord Shiva into three pieces. Well here is the story of what actually happened .
In Vamana Purana , Sage Athreya told Upamanyu why Lord Vishnu Cuts Shiva into three pieces and how Lord Vishnu got the Sudarshan Chakra in his hands.Later same story is told to Narada by sage Pulastya.
Shridama want to steel Lord vishnu’s Ornament
In Ancient times there lived a demon named Shridama who ruled the entire earth. Once demon Shridama had an eye on Lord Vishnu’s dearest ornament “Srivatsa” and want to take it by any means. Lord Vishnu came to know the intentions behind Demon Shridama and wanted to fight him. But Lord Vishnu had no weapon by then and went to Lord Shiva for help. Lord Shiva gave Sudharshan Chakra(Wheel) to Vishnu and told he can kill any body with that magical wheel weapon.
Lord Vishnu test the Sudharshan chakra on Lord Shiva
Lord Vishnu then wanted to test this wheel weapon to know how it works. Lord Vishnu wanted to test the wheel on Lord Shiva himself and expressed his desire. Lord Shiva agreed to test on him. Lord Vishnu then released the Sudarshan Chakra towards Shiva and cut him into three pieces. Each of that piece is named as Vishvesh, Yagnesh and Yagnayajak.
Note: Lord Shiva later joined himself with his Tantric power.
Lord Vishnu then satisfied with the weapon, thanked Lord Shiva and proceeded to kill Demon Shridama. Lord Vishnu went to the mountain where the demon Shridama lived and severed his head with the Sudarshan chakra. After killing the demon, Lord Vishnu returned to his abode Ksheersagar.