>If used in succession with other protection moves, its success rate is cut to 33% of what it was, working cumulatively the more it's used. This will prevent damage and effects of almost all possible attacks that would be inflicted upon the user.

-Serebii

Bulbapedia for some reason doesn't have this. But this is right because even PS! Uses this description.

>This move has a 1/X chance of being successful, where X starts at 1 and triples each time the move is successfully used.

-PS!

And using 3 times in a row the chance for it to fail is 1/9 which is 11.11%