I have to disagree...I'm not sure I would put an Oto in a 3 gal, in the first place. *That's an awfully small tank, even for a Betta. *Otos are very sensitive to water quality and it takes a very experienced fishkeeper to keep perfect water quality in such a small tank. *And watching how Mist and Marble swim around together, I'm not sure how happy they would be on their own. *Rose's situation with an Oto on his own is a different situation than just going out and taking only one Oto away from all his friends at the store. *Also there's the fact that you're going to want to get your Betta into the bigger tank as soon as it's cycled...not sure you want to wait another 2-4 months to make sure the Oto you put in is not sick. *(I would not recommend putting anything in after the Betta since in that size of a tank there's not a lot of room for the fish to get away from the Betta if he's picking on them).
Well that's just my opinion...you should do whatever you think is best.
Oh, I see you have posted again Miley, I typed this before I saw you're second post. To answer your question...no there is no way to prove that a Betta will get along wiht anything. There are some that are perfectly peaceful while others are bullies. Otos seem to have the highest success rate with Bettas than anything else. But that is another concern of mine...if the Oto is being bullied, although I doubt that would happen, you don't have anywhere to move one of the fish as far as I know (or do you have a q tank? Sorry I can't remember)