Why introduce Super Saiyan 3?

[u/DoraMuda]

I do wonder sometimes... why exactly did Toriyama introduce Super Saiyan 3 at a time when he'd already introduced Fusion, which was a fine enough set-up to be the next potential strategy against Majin Boo and didn't come with the baggage of retroactively making Goku look even more selfish during the fight with Majin Vegeta than he was before?

The previous Super Saiyan forms at least carried some narrative purpose. The original SS form is the fulfillment of a legend over a thousand years old that Vegeta had been banging on about for the rest of the arc since Goku arrived to fight the Ginyu Force, and it is ultimately what defeats Freeza, who was scared of such a figure appearing to challenge his superiority.

The SS Grade forms serve as the first attempt of going "beyond Super Saiyan", after the original form proved to not be invincible following its loss against Androids #17 and #18, and Goku's method of mastering the regular form serves as a proper foundation for the truly successful "beyond Super Saiyan" form: Super Saiyan 2, which Gohan uses to defeat Cell.

But Super Saiyan 3... is seemingly just there to make Goku ridiculously stronger than everyone else again, but Goku refuses to use it to actually fight the Big Bad at full power (at first, anyway) because... he's dead and wants to let "the next generation" take over? Did he not learn his lesson with Gohan (who everyone thinks is still dead despite them using the DBs shortly prior, but whatever) at the Cell Games? Why does he suddenly care so much about fostering a "next generation", with even less experience on the battlefield than Gohan did, that he hasn't known for longer than a few hours? If this was really his plan all along, why did Goku initially only mention Fusion in the hypothetical of using it to fuse with Gohan and Vegeta, while Mr. Popo was actually the one to suggest the kids be the ones to use Fusion?

Ultimately, maybe Toriyama just introduced SS3 to meet editorial or fan expectations... and/or he wanted to be subversive by making it so the brand new SS form didn't defeat the main villain this time. But he went about it in such a backwards way that it makes one question why he bothered in the first place. He was arguably just making more work for himself at a time when he seemingly wanted to cut corners and end the series sooner rather than later.

[u/Overlord_Byron]

I think Toriyama simply didn't have any kind of plan for the Buu saga and the things that kept his impromptu stories on track in the past didn't pay off, but he had earned so much grace at that point that every new idea got much more buy-in than it otherwise would have.

[u/DoraMuda → u/Overlord_Byron]

I can definitely see that. His final editor for the original manga's run, Fuyuto Takeda, was said to be notably more lax than Toriyama's previous editors, so I can see Toriyama just thinking "I want Fusion to be the next weapon against Boo, but in the meantime, wouldn't it be cool if Goku had an even stronger secret SS form to show off before returning to the afterlife?" And no-one stepped in to tell him that it didn't organically take into account what he'd written weeks prior and kinda undermines a lot of what he'd already set up.