It'd be incredibly boring in 3D.
1. Using your butt is a downright unviable tactic. Real humans aren't as flexible as they are in Keijo, so using your butt not only means exposing yourself, it's also a highly ineffective method of attacking.
2. Boobs are the only viable means of attacking. Which means that what you effectively get is a boob-centric shoving match. Like sumo wrestling except neither party has a reliable means of tackling the opponent.
3. Women cannot fight, they simply cannot fight. Even UFC eyecandy can't really fight which is why even miss popularity Roussy refuses to fight men. And Roussy is only popular because she "fights like a man" against people who fight like teenage girls.
Keep keijo fun, keep it 2D.