On the darknet, they exist. I'm not sure about their reliability.
Yes, they exist, and most, if not all, of them are crypto casinos due to the level of anonymity that cryptocurrencies provide.
However, I would never recommend anyone play on one of them because of the high risks involved.
If your account gets banned or you’re unable to withdraw your money, no one will be able to help you, as these casinos are usually unregulated and their owners are anonymous.