Goten and Buu have fought a few times in the past. As Goten he’s hopelessly outmatched, but in SSJ3 Gotenks form he’s not a bad fighter. Still, I wouldn’t say he’s quite Buu level. Buu has the power of Mystic Gohan and SSJ3 Gotenks at his disposal! Not to mention Piccolo since he still helps in the end.
Gotenks SSJ3 is strong, but another advantage that Buu has is his regeneration. He can regenerate many times. So while Goten starts to get tired, Buu will still be up and kicking. Goten is strong, but I’m afraid he just couldn’t win this round. Buu wins.