One of the most powerful combat transformations unveiled in all of Dragon Ball Z is Super Saiyan 3, achieved by Goku and Gotenks near the end of the series. Discovered by Goku while he was training for seven years in the afterlife following his sacrifice at the hands of Cell, it was this transformation that allowed the mighty Saiyan to stand toe-to-toe against Majin Buu and ultimately triumph over the magical supervillain in his evil, final form.
However, given the strange, esoteric nature of the transformation -- including the fact that no one appeared to even know about it until Goku returned to Earth with the form -- this begs the question of whether Goku would have attainted Super Saiyan 3 if he hadn't learned about it while dead.
In comparison to Super Saiyan 2, Super Saiyan 3 is not only a drastic increase in power but also in physical appearance -- users develop increased muscle mass and a pronounced brow at the visible cost of their eyebrows while radiating even more raw energy. Taking a bit more time to charge up and consuming energy at an accelerated rate, Goku and Gotenks remain the only two characters to canonically achieve Super Saiyan 3 in the Dragon Ball franchise. Goku taught Goten and Trunks how to reach the form with the enormous potential provided by their fused state, but he had to return to the afterlife sooner than expected due to Super Saiyan 3 consuming his remaining time on Earth.
With all the other Super Saiyans -- including Vegeta and Gohan -- unable to achieve Super Saiyan 3 even into Dragon Ball Super, it stands to reason that Goku's training in the afterlife made his own achievement of the form possible. After being resurrected by the Elder Kai, Goku had trouble maintaining the transformation, reasoning the added endurance and energy his body possessed while dead was better suited to handle the Super Saiyan 3 form. As such, Goku only uses Super Saiyan 3 sparingly in Dragon Ball Super and, while Vegeta and Gohan likely have enough power to achieve the form if they chose to in the anime, both choose to avoid it entirely, with Gohan sticking to his ultimate form while Vegeta transforms into a Super Saiyan God.
The exact nature of how Goku discovered the transformation remains something of a mystery. Yet it is heavily implied that had he not spent seven years rigorously training in the afterlife, he would never have discovered Super Saiyan 3 at all. Everyone is incredulous about the possibility of such a transformation existing, and Vegeta doesn't even learn about it until he arrives in the afterlife himself. And given that Goku has to put so much strain in reaching the power necessary to initiate the transformation -- even in his heavenly body when first unveiling the form against Majin Buu -- this all suggests Goku needed to travel to the afterlife to learn about and achieve the transformation in the first place.
Even in the face of transformations like Ultra Instinct and Ultra Ego, Super Saiyan 3 remains one of the stranger combat transformations in the Dragon Ball franchise. While only occasionally seen in the story since the end of DBZ, little information about the form's origins and full nature has been revealed. Given how mysterious and difficult the transformation is to achieve and maintain, it took Goku that prolonged time in the afterlife to discover it for himself as a visible reminder of his seven years dead after fighting against Cell.
