Instead of Vegeta exploding he got SSJ3?

[Pocket-God]

So instead of Majin Vegeta using Self Destruct against Buu he somehow pulled SSJ3 out of his ass, what changes?

 

[Papasmurf]

DBZ becomes DBS.

 

[Captain Cadaver]

He'd just end up draining all his Ki before he could kill Boo and just dies again. This changes the battle against Pure Boo a lot though, since a dead SS3 Vegeta would be able to kill Boo as soon as Galu wastes his Ki. Therefore, there's no return of Good Boo or Vegeta acknowledging Goku's superiority, nor will Oob exist unless Enma makes the decision himself so as to not deal with the possibility of Boo wrecking his office.
As for Super, I doubt this would change the plot so much. Whilst Beerus probably wouldn't try to destroy Earth if he got his pudding, Goku would probably still confront him not long after the party with the idea of using the Dragon Balls to learn about the SSG. After that, the plot continues as normal seeing as how Boo plays no significant role in later events. I doubt Vegeta not getting his rage boost would affect his progression much, considering he could just do the God ritual himself and training gains are plot related anyway considering how strong most of the supporting cast got (Piccolo, Kuririn, Gohan, #17, etc.)

 

[Ultimate Cell]

He would smash Buu like Play-Doh and kill him.

 

[Void]

He's too wounded from Fat Boo at that point and SS3 takes a huge toll on a living body. At full power SS3 Majin Vegeta could beat Fatty, but in his condition here he can't win.

 

[SSJ2]

Pretty much what Paladin said. However, Goku seemed to get restored to full power in his first SSJ transformation, so perhaps similar could occur here.. Though I find it doubtful considering how much vitality SSJ3 drains.

 

[theallpowerfulpuipui]

DBZ becomes DBS.

 

[Zippy]

Speaking plotwise a new transformation means an assbeating so hed likely fuck him up but its too early in the saga for buu to die completely so hed probably regeneration hax his way back into the story like the original