I think I know who the third victim was, I don't know if anyone has said this before because I left the manga brew for about 30 chapters before coming back now.
So my theory is based on: 1. Yuka's lie 2. The third victim being a murder, but with administrators implying it wasn't a direct one 3. Sawaragi senior and Tenji calling it a tragedy 4. Yuuichi saying it was entirely for selfish reasons and wanting to unhear Yuka's secret
I think the lie Yuka told her friends was the only thing we really know about her, that she was unable to have children. She was pregnant at the time of her death, so the baby died before they were born. This makes it an indirect murder, since the victim died indirecly because their mother was killed and because they didn't get to be fully alive, therefore making it a "doubtful" murder. Yuuichi wanted her to be his family, to be her only son, and was about to be "replaced", he was already feeling betrayed and now doubly so. He'd have prefered not knowing this. This is already a tragedy, but we can take it further. Why would Yuka, who could barely maintein herself, go for a random adoption? Because Yuuichi was probably her biological son. Double tragedy, as that would mean that the mom he killed was his "real" mom, and the baby was his sibling. Detective Sawaragi would know this thanks to the autopsy. Yuuichi was a kid and did on impulse, so regret drove him to try to be a better person and maybe even see the kids of her mother's friends as a compensation to protect instead of the younger sibling.
Sorry it was so long, thoughts?
Uuff... I cried. I want Mikey to be happy too.
Anyone noticed the importance of Takemitchy calling him Manjiro? "Mikey" was the leader, he had to be strong por everyone, it was a name that carried responsability ever since he took it to make his little sister happy, and then made it into a symbol of his Toman persona. "Manjiro" on the other hand is how his brother called him. Who he was from birth, the child, who could allow himself to depend on others. The very name is telling him he doesn't have to be strong, he deserves to be protected too.
And then... Finally in the current present, he has called Takemichi "Takemitchy", which is the name he gave our MC when he saw him, saw his brother in him and thought "Ah... This person is someone I can trust".
I just loved it.
I hope this follows the "Beware of the villainess" route where she lectures all "original love interests" and takes matters in her hands while beating some sense into them and meets and actual green flag meanwhile ╮( ̄▽ ̄)╭