Man, if I could guarantee the same results, I’d get another tank just for this purpose. I feel sorry for betas, they get a raw deal..solitary confinement in tiny tanks.
Anybody can do this. Pick out your betta's put them side by side and see if they flare at each other. I picked out 2 and liked how they were. I had 1 younger 1 already. All I did is float them in the 10g for a day and night in a cycle so they see each other. Next day released them same time. Watched carefully that day and next day. This takes time and patience. When ever they flared or started fighting I cupped the one who started it, floated her for a day and over night, then released. I did this with all of them at one point and time. It works! It gives them the thought of living in a cup or in a place where they can swim. So, if anyone thinks they can't do this. Please reconsider. I didn't post this part because at the time. I had alot of things going on. But you all must know I had some tails spilt but they where very miner and healed right up the next day. Only because I watched very carefully for a week. They all get along very well now. But, also keep in mind this is only a 10g. The idea for 3 betta's is 15-20g. But I took a chance. Anyone who has done this probably wouldn't tell you what they had to do to get this to work. And I'll also add, the younger you go the better and easier it will be. My blue betta is older than my other 2 and I had a little more problem with her than my younger ones. All you gotta do is watch them closely for a week. After that if done the way I did it, it will work.