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Well usually casinos offers instant deposits and you can start playing right away.They only want a few confirmations before withdrawing and this amount is usually 3 confirmations.In case you win you just wait for the 3 confirmations and then can withdraw instantly.
As far as I know, in order for you to get and be able to "use" your withdrawals, you will be needing more than 3 confirmations from the blockchain of bitcoin. As I understand it, it is different from instant deposits in which most of the time 1,2, or 3 confirmations are enough to have the deposits credited to your casino wallet.
3 confirmations are just the minimum requirement, probably the standard, but in terms of deposit, 1 confirmation is already enough, a gambler can already see his deposit and he can start gambling right away, that's a marketing strategy of a gambling site as they want to attract more gamblers to play.
Even an unconfirmed transaction as long as it was already visible on the blockchain, Casino already credit the deposit just like on sportsbet, FJ and PD but some casino like bitsler has some special condition like checking the trust score first of the account in able to credit deposit without confirmation since double spending on Bitcoin is a bit not possible now on the current code.