Tragically, people seem to think that Kid Buu is stronger than Super Buu 3; this is much like claiming that Nappa could beat Perfect Cell.
In the manga it's made very clear that Super Buu - without any absorptions - is much stronger than FPSSJ3 Goku. While inside Buu's body, after removing the pods, Vegeta says: "Now, let's blow a hole and escape from here", only to have Goku reply: "Even though his power has gone down, he's too strong for both of us. This is your fault for destroying the Potara!" (Potara = fusion earrings, for the dubbites in the audience.)
Goku is stated to be able to kill Kid Buu with ease at his full power SSJ3. Vegeta tells him "With the full power of SSJ3, you'll beat him certainly!", and both Goku and Vegeta relax and even smile when Kid Buu emerges from Buff Buu, who they were terrified of.
With this in place, it's obvious enough that Super Buu 3 is leagues beyond Kid Buu. However, let's not stop there with the proof. Super Buu 2 says "I am the mightiest Majin!", and as Super Buu later proves inside himself as Vegeta reaches for Mr. Buu's pod, he is well aware of the existence of Kid Buu. Why didn't Super Buu say he was the mightiest Majin? Because Buff Buu is stronger than Super Buu, as he has none of Dai Kaioshin's influence.
When Goku arrives on Earth to help Gohan fight Super Buu 2, he's not doing it by going SSJ3 and beating him, as you mostly seem to think he can. No, he's there to fuse with Gohan. Why? Because Super Buu 2 is far, far, far beyond Goku, and as shown, is stronger than Gohan as well. Super Buu 2 is already shown without doubt to be stronger than Kid Buu.
Later on, when Super Buu 3 is born, Goku is considering fusing with Mr. Satan/Hercule. And you think Kid Buu is stronger than Super Buu 3? When Goku refused to fuse with Vegeta to fight Kid Buu, but was a second away from fusing with Mr. Satan PERMANENTLY to fight Super Buu 3? There's no way you can deny that Super Buu 3 and Super Buu 2 are stronger than Kid Buu. To do so is to ignore the obvious.
When fighting Fat Buu, Goku admits to Piccolo that "Majin Buu's power is incredible. I'm not sure I could have beaten him." Compare this to his later reaction against Kid Buu, where he's confident he can beat the stuffing out of him. However, he then tries to power up to the level he used previously, and SSJ3 fails. He realises he can't go FPSSJ3 while dead, and now the level that he wasn't sure could beat Fat Buu is unavailable to him - yet his weakened SSJ3 goes dead even with Kid Buu. Kid Buu looks less powerful by the minute, doesn't he?
Kid Buu faces off against Mr. Buu, the pure-good fat Buu who lost most of his power went splitting from Fat Buu. In the manga, it's said "Most of the power went to the Evil Buu", and given the beating Mr. Buu got from his grey counterpart, I'd say it's pretty clear Evil Buu got the vast majority. Yet Mr. Buu is able to fight Kid Buu long enough to give Goku time to form the Genki Dama. Evil Buu took down Mr. Buu in a few minutes. Evil Buu and Mr. Buu are both parts of Fat Buu. These two combined are Fat Buu. Kid Buu can't be stronger than Fat Buu.
Now on to another fact that people get wrong. So many people say "When Super Buu 2 (Buu w/Gotenks+Piccolo) defused to Super Buu 1.5 (Buu w/Goten+Trunks+Piccolo), Goku said that he could beat him!" This is both the dub and misunderstanding at work. The original line is: "Gohan can beat you now himself, without the Fusion." Goku says nothing about fighting Super Buu. Seeing as he says he can't beat Super Buu without any absorptions whatosever, I highly doubt he can beat Super Buu with Piccolo, Goten and Trunks. And he never claims that he can.
The correct power of Majin Buu:
Super Buu 3 (Buu w/Gohan/Goten/Trunks/Piccolo)
Super Buu 2 (Buu w/Gotenks/Piccolo)
Buff Buu (South Kaioshin dominant)
Super Buu 1.5 (Buu w/Piccolo/Goten/Trunks)
Super Buu (Buu w/Dai Kaioshin influence reduced)
Fat Buu (Buu influenced by Dai Kaioshin)
Evil Buu (Pure evil from Fat Buu)
Kid Buu (Kid Buu's original form, created by Bibidi)
Mr. Buu (Pure good from Fat Buu)
Okay, now to handle the inevitable people who think the anime or the dub are somehow better than the manga.
The manga is to Dragonball like the Lord of the Rings novels are to the movies. You can go watch the movies and enjoy them, but don't think that the additions and removals became fact. Legolas never did take down an elephant acrobatics-style in the Lord of the Rings, it was just in the movie for added kicks. And that's where Toei comes in.
Akira Toriyama's manga was adapted by Toei as an anime. Toei, needing to give Akira Toriyama space to draw, added "filler" from their animators to close up the gaps. Much of the filler is simply the extended staring and twitching that happens when two fighters are battling near a bunch of weaker ones. Some of it is various things the animators slapped together. This filler is what confuses you all in regards to Majin Buu.
Some scenes added by the animators include:
* Goku fighting any form of Super Buu
* Goku saying "You're much stronger than all the other Majin Buus" to Kid Buu
* Tenshinhan, Yamcha and Chaotsu beating the Ginyu Force, each one far stronger than Vegeta on Earth, after only having trained on Kaio's Planet for several days
* Vegeta fighting Gotenks and Piccolo inside Buu's mind
* Goku fighting Gohan inside Buu's mind
* Vegeta destroying Arlia with his finger
* The Heavens Tournament, featuring Pikkon and his incredible ability to beat Super Perfect Cell in one hit
* The Garlic Jr Saga
Basically, if it doesn't make sense, like the above, it's because Akira didn't add it. A bunch of Toei workers did. Toei don't know Dragonball, they're just there to make it into an anime. When they don't have enough manga material they make their own. This makes for entertaining viewing but NOT for facts.
Finally, the dub. Ah, Funimation. It should be obvious that the dub is no source at all. Here's just a select few mistakes the dub made, and there are a lot:
* Vegeta claims that Bardock was a scientist
* Goku decides that Vegeta was the one who stepped on his grandfather
* Frieza was using 1% power against Goku (it's said to be 33%)
* Recoome being an android
* Goku realises that Vegeta was only "bad because Frieza made him be bad" - in actual fact, he said that he'd hated Vegeta but respected his Saiyajin Pride, but that's too hardcore for Funi
* In response to "Does that mean Gotenks is stronger than you?", Goku saying "No, I can do that too", when the original lines were "Is that Fusion?", "No, that's SSJ3! The boys have completedly mastered it! I'm impressed!"
* Goten saying that Super Buu was equal to Gotenks, even though they'd just beat the crap out of aforesaid Super Buu
* Kaioshin saying "This is the strongest Buu of all!" in regards to Kid Buu
Do I have to go on? You get the picture. Toei and Funimation know just as much about Dragonball as the next person you meet on the street. Akira Toriyama invented Dragonball. He IS Dragonball. If his manga says it, it's fact. If Toei's animators and a guy at Funimation who translates from Japanese and then changes things around say it, it's not fact. And the manga says this:
Kid Buu is nowhere, not even CLOSE, to Super Buu 3.
This is the end of my long message.