My understanding is that it was a poly relationship, simply a friends with benefits where all of them knew that he was doing it with other people as well but who knows. There's gonna be a volume 2 otw anyway so maybe we'll know then
https://www.mangago.zone/read-manga/tameshi_ni_komando_itte_mita/uu/mk_v-1-chapter-3/pg-31/
As I understood it, this implies that his other two partners offered the same. They can clearly see Masa is getting more fulfillment out of Oto (though Masa doesn't outright mention his name) and all 3 offered to stop the mutual dom/sub relations. I think it's implied that it ends with them from here. A lot could have been missed in the translation but that's how I took the scene.
they got tgt but masa is still in the poly rs with other 3 people?