Sometime after defecting from Konoha, Hiruko assembled his own team. He trusted them enough to teach them his Chimera Technique. They're responsible for keeping unwanted outsiders from reaching Hiruko, protecting the perimeter of Mount Shumisen, as well as fighting those who reached the gates on the way to the mountain peak. When Hiruko tried to collect Kakashi's Sharingan, each of them fought in one gate, Ichi guarded the first gate, and fought Team Guy, Ni guarded the second gate, and fought Team Kurenai, and San guarded the third gate, and fought Team Asuma, with Hiruko welcoming Kakashi after the fourth gate. All three had a mark on their backs, similar to the mark on Hiruko's hand. San later summoned the recently defeated Ichi and Ni, and merged with them, forming a chimera beast, an union of the animals each controlled independently. After being restrained by Team Guy, Team Kurenai, and Team Asuma, it was summoned by Hiruko, and was subsequently destroyed by the combined efforts of the Konoha 11.
The names of the three members are all numbers: "Ichi" means "one", "Ni" means "two", and "San" means "three". This is represented by the number of stripped marks on their faces, as well as the gate they are guarding in sequence.