I think that n%3 mean that a number divide by 3 and what you want is the remainder. n//3%3 mean that you are diving the number dirst and rounding it down, then you are doinging another division and looking for it's remainder. Same with n//9%3 This just mean that you are dividing by 9 first and rounding down, then you divide by 3 and get the remainder.
For example;
5%3 would have a remainder of 2.
5//3%3 would be 1.66 and round down would have 1, then you find the remainder which is 1.
12//9%3 would be 1.33 and round down you get 1, then you find the remainder of 1/3 which is 1.