No, Goku likely could not have defeated Fat Buu definitively.
While Super Saiyan 3 Goku demonstrably outclassed Fat Buu in terms of raw power, Goku himself acknowledged that he was likely unable to defeat him. He explained to Piccolo that Buu's power felt like a "lie," suggesting an unpredictable and ever-evolving potential. Furthermore, Goku was already pushing his limits maintaining the SSJ3 transformation for the duration of the fight, which would prevent him from drawing on its power further as the battle continued. Given Buu's regenerative capabilities and ability to draw power from anger, Goku recognized that the fight was ultimately unwinnable for him at that time. He felt the only hope of stopping Buu was for Goten and Trunks to fuse.
