Yeah, its a mythology joke about Omoikane, Tamayori, and Toyotama.
As for the BBA, while the simple answer is that Junko just doesn't act like an old lady, there's also the fact that the original BBA was based on a pun in their names. Yukari, Eirin, and Kanako all have the kanji 八 in their surnames (the 'ya' in Yakumo, Yagokoro, and Yasaka). They also happen to act older, and "old hag" can be written as ババ, which you'll note looks awfully similar. There's obviously no hard and fast rule for this, and Byakuren became an honorary member at some point later on, but the point is just that it originated as a specific joke, not just an arbitrary collection of characters.
But yeah, Junko doesn't act like she's all that old. In fact, she seems somewhat childish and is often portrayed as such, despite explicitly being a mother.