Well, since you have 12 fish in a 3g, that gives the betta about 1/4 of a gallon of water, which isn't even accurate because the sizes of fish are different.
So theoretically, right now, you are keeping a betta in a cup...don't the cups usually say on them that this should not be their permanent home? Yeah, bowls suck and shouldn't be used, but don't bash pet stores when you are basically doing the same thing.
And what do you mean with "they are forced to live alone?" Males CAN NOT be kept together. And all bettas are sold separately. Even keeping males and females together is sometimes a problem. It depends on the fish itself if it can live with others. Bettas can be community fish.