Multiple male bettas in the same large tank without dividers can work ... until it doesn't. I've read many stories of people who attempt this and it looks like it is working, only to have it end in dead fish after a few weeks or months of uneasy cohabitation.
Juvenile males can be kept this way while young but the adults will get increasingly territorial, aggressive and stressed out by the presence of other males. You can try utilizing similar tactics as setting up a tank for aggressive cichlids. Lots of hiding spots or floating plants and other fish to distract the bettas does seem to help. Interestingly, two bettas alone in a big tank are more likely to death-match fight, compared with a larger group of bettas. They claim smaller territories and fight more frequently but less seriously due to the presence of multiple threats.
Personally, I don't recommend it. Divided tanks are safer.