Goku can predict the results of the fusion just from seeing the two individuals. If he couldn't do that, he never would have left this to them in the first place. But as it stands, SS Gotenks [pre] > SS3 Goku is stated right there in the manga, with nothing to contradict it.
He directly said they would easily crush Buu, and that Gotenks would be stronger than him.
Nothing contradicts Super Saiyan Gotenks < Super Saiyan 3 Goku either. He really had to leave it up to them because no one else besides him could beat Buu. That and he wanted to leave it up to the new generation to save the earth. But like I said, that was before they fused. He said, "a warrior stronger than me" would beat Buu, but never specified the form. Keep in mind, this was before ANYONE, including Goku, knew that they could even transform to Super Saiyan in fusion. They even experimented with it later to see if it would work. From what you're saying, Goku would've already knew that they could be able to transform in their fusion, even though NO ONE knew they could even do that, which is kind of wonky. Goku's not psychic, unlike his father. For all we know, Goku could've predicted that Gotenks would beat Buu in his regular fused form, which he would then be wrong considering that Buu handed Gotenks his own butt when he was in base. You could really go both ways with this.
And to be frank, all of that is speculation anyways because not everything about Gotenks (or the entire Buu saga for that matter) is clear-cut. So saying that it is, "directly said", in the manga is far from the truth, because if that were the case, we wouldn't have countless "Gotenks" debates all over the internet because it would be sooooooo "directly said" or "clear in the manga". Regardless if he's stronger than Super Saiyan 3 Goku or weaker, it still works because the end result was still the same. He turned Super Saiyan 3 and served no other purpose to be absorbed by a horrible, gelatinous, pink monster

.
Also he never said that to Buu.
What are you talking about?
He stated clearly to Fat Buu that a warrior stronger than himself would fight him in a couple days. That right there is clear as day.
