SSJ3 Goku (in Other World body) VS Pre-Split Fat Buu (with SSJ2 Majin Vegeta and Piccolo absorbed)-- no candy beams or any more absorbing
Hmm... hard to say. SSj2 Vegeta should be about a quarter of SSj3 Goku's power since they were evenly matched in SSj2, and Piccolo shouldn't be too far off from an eighth. Buu can't be too close to SSj2 Vegeta considering how badly he beat him, and how he managed to survive his final attack (which should be way more powerful than what Vegeta's level would've been during the fight leading up to it)... so it all depends on just how big of a gap there is between Fat Buu and SSj3 Goku to begin with, I guess.
SSj3 Goku = 8
Fat Buu = 5
SSj2 Vegeta = 2
Piccolo = 1 (...okay probably closer to 0.75 or 0.6 or something, since he's weaker than the adult Super Saiyans, but let's round up just for convenience)
Assuming their power levels compare like that, the resulting "Super Fat Buu" would be an 8 on that scale (or slightly under considering Piccolo being weaker than the Super Saiyans.) So yeah, Goku would be able to win but it would be
tough, very similar to his battle with Kid Buu, he'd need to power up to his maximum for a huge blast that would completely destroy Buu with no possibility of regeneration.
If Fat Buu's any closer to Goku's level than that, Goku might not be able to pull it off with just the "SSj3+huge full-power blast" method. A massive Genki Dama (or waiting for Ultimate/Mystic Gohan or SSj3 Gotenks to show up and finish the fight for him) would be the only way.
Base Young Future Trunks (post-Gohan training but pre-SSJ transformation) VS Cell Games Krillin, Tenshinhan, Yamcha, and Chiaotzu (No Tribeams or Kienzans)
Tribeams and Kienzans?

That wimpy version of Trunks' only chance is if you don't even allow the humans to use ordinary punches and kicks, and even then he might hurt himself punching Tenshinhan and Krillin before they poke, slap, and chest-bump him into submission. One of those two beats him one-on-one with basically no effort while Yamcha, Chiaotzu, and whichever of the two bald guys isn't busy beating the crap out of Trunks fly to the nearest city to get some take-out for everyone's lunch once this joke of a fight is over.
If it's TV Special version (weaker than the Gohan who's weaker than Goku was vs. Freeza, but not to an absurd degree) rather than manga version (so weak his Super Saiyan self is on par with Gohan's base form), I'd still say that Krillin or Tenshinhan can beat him easily even without using their super-moves. I think those two at least kept up with the base Saiyans through the beginning of the Android saga (which would all be much stronger than that Trunks), only being left behind when the Saiyans all started getting massive boosts from "year-in-a-day" training and mastering Super Saiyan. Yamcha, however, would still probably lose (though not without putting up at least a bit of a fight, probably catching him off guard with the Sokidan or something) and Chiaotzu wouldn't be able to do much.
Android 13 VS Android 19 and 20 (no absorbing)
#19 and #20 have no chance whatsoever against #13 if they can't absorb his energy. #13 seems to be a good bit stronger than either #14 or #15 considering that he didn't get overpowered when Goku went Super Saiyan unlike the other two--and even #15 wasn't too far off from Vegeta (managing to knock him out of Super Saiyan in their final clash when #15 got destroyed), who could easily destroy either #19 or #20 if they didn't drain him too much first (and proved he could easily destroy #19 even
after being drained.)
Honestly #19 and #20 probably couldn't pull it off even if they
were allowed to absorb energy, unless #13 did something really stupid like firing loads of powerful ki blasts at them both before even trying to attack physically.