Either 3 or 5%, still, the percentage is low considering the number of gamblers worldwide. It means, casinos are indeed earning good amount of money. Hence, this type of business is really a lucrative one. Whether you play casino or slots, they are relying both in luck. This is why, no one can guarantee such winnings on this type of gambling game.
Please, note that 3 or 5% of winners doesn't mean 95 or 97% of profit for casinos. If you look at my post above I explain the reason why it is not the same, although many people think that way.
In addition, the number of winners can be way higher depending on where you look. In some oficial statistics in my country I've read that the number of winners rises to 17%.