I doubled Goku's power from the Cell Games to the Buu Saga and I have them as equals so this would result in Buu winning.... We are talking about Pure Evil Majin Buu aka the Grey one right?
Even if this SSJ3 Goku is close to Buu (I don't think he is), I always follow my own rule that someone should be at least 1.5x a Buu in order to destroy it. Anything close to even and all of Buu's abilities end up winning out.