There were numerous defections from Ali's camp in the aftermath of the Battle of Siffin.
A number of them met in Mecca and discussed the 659 Battle of Nahrawan, which took place as a consequence of Siffin, wherein most of their men were eradicated while facing Ali's army.
The assassination attempts were to occur simultaneously as the three Muslims came to lead morning prayer in their respective cities of Fustat, Damascus and Kufa.
The method was to come out of the prayer ranks and strike the targets with a sword dipped in poison.
[4] On 26 January 661, while Ali was praying in the Great Mosque of Kufa, Ibn Muljam struck him with a poison-coated sword.