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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.