Allen: If it's just a random body, SCREAMS TO HIGH HEAVEN and freaks out. Cue incredibly comedic two chapters of the manga where it turns out to be due to a science department potion and no one is actually dead. He would be very honest and very alarmed and get help with the body and try to figure out what happened but be so, so rattled.
Flynn: Yelp and fall out of bed. He's a little more circumspect and he doesn't tell just everyone; only getting a few people involved to try and figure out what's going on. He'll deal with it quietly and be very, very, very displeased about it. Someone would get their ass arrested or punched.
Roxas: Scramble out of bed, try to figure out where it came from, who it is. He'd be less concerned about the potential murder and more concerned about what the body was doing in his bed. He would probably make a network post going "What's this all about?", relatively calm.
If you had to pick up at least one of your dropped characters, who would it be and why?