Triple Throw 30.

and it works just by increasing probability after failures.

No, it doesn't. Pseudo-random generators just use hash methods that are most evenly distributed, so they'll always have the same probability no matter what happened previously. The only way to predict what they'll do is to perfectly imitate the system & input they're getting, and run the algorithm yourself. Since you don't know what algorithm nexon uses, what it's being used for (just scrolls, scrolls of a specific type, scrolls on only 1 channel, etc.) you have very little chance of successfully predicting what will happen.

Working in some sort of "if you fail 6 times, next is more likely to work" would be way more complicated than the simple "generate a random number every time".