There is no requirement for hitting SSJ3, it's just having the power to reach and maintain it. The reason Goku was able to do it in Other World is because he was dead and didn't have the limitations of a physical body. Gotenks is able to do it because he is a fusion and the power of Goten and Trunks is combined and enhanced. Vegeto would have been able to do it against Super Buu but didn't cause he didn't need to. Gogeta could have done it in the Fusion Reborn movie, and against Broly but didn't need to. SSJ2 was the best option at the time for strength and ki usage. And the line from the Yardrat was during Vegeta's training to learn Spirit Fission for the fight against Moro. It had nothing to do with him using SSJ3. There has never been a theory to how each new SSJ form is reached. We have known how they did it for decades now.