From Bite Marks to First Love (Arinko)

"We slept with you last night."

An invitation to dinner from the Omega he fancied...
Overjoyed, Naru drank until he passed out—and while he expected to wake up to a refreshing morning, he instead found his friends, Masa and Sogo, lying on either side of him. Naru had always maintained a wholesome, platonic friendship with the two of them as fellow Alphas; yet, for some reason, on this particular morning, all three of them were stark naked—and a dull ache throbbed in Naru's backside.

Naru, who had always coasted through life in "easy mode," suddenly found himself ambushed by a love triangle of epic proportions: Alpha x Alpha x Alpha!

Original synopsis found here

Note: originally serialized under the same title as Kamiato kara, Hatsukoi as chapters 8-14