The trio are a group of friends all wearing Maromi back packs introduced in episode 8. They are not in the openings or closing credits of the anime.
Flashbacks to their online conversations reveal that they may be some kind of suicide club. The two older men are very surprised that Kamome turned out to be a little girl though, and ran away from her.
They considered subway as a means after carbon monoxide failed, but someone beat them to it.
During the episode, trio goes throught many adventures on their way to die which is on it's way bonding.
At one point they face Lil' Slugger what gives them hope for quick, relieving death.