The 3rd floor trashroom is locked for me, do you know how to unlock it?
It's always locked. When you go there in the afternoon after Misa comments about hearing a noise in there, you'll encounter the Sakurako scenes (you also have to do the quest line with the Black neighbor on the 3rd floor first, the guy who was trying to sleep with her).
After that you'll see a heart icon in the afternoon outside the Supply Closet on the 3rd floor, and get the option to peek on Sakurako doing the deed. First is at Chaos 5-24 which is her having sex with her boyfriend. Then after 25+ she's cheating on her boyfriend with the Landlord cuz he has a bigger dick. Then finally at the final tier the Black neighbor has his turn with her.
NGL as the player *I* feel cucked watching that cuz despite it being a netori game you got this one girl who is completely unobtainable and sleeping with the scummiest loser you got to deal with every week, the Landlord.