A, A, B, B, C, C =6! / 2!.2!.2! = 90 permutations.
Each permutation will begin with one of letters in: [90 / 6] x 2 =30 cases for each of the 3 letters. And in 4/6 of the cases at least two of the same letters will be next to each other. And in 1/3 of the cases, they will not be. So, for each permutation beginning with A, you will have:
1/3 x 30 = 10 permutations when they will NOT be next to each other.
So, you will have a total of: 10 x 3 = 30 permutations where the 3 letters will be separated from each other. Here are just 10 of them beginning with the letter A:
{A, B, A, C, B, C}, {A, B, C, A, B, C}, {A, B, C, A, C, B}, {A, B, C, B, A, C}, {A, B, C, B, C, A},{A, C, A, B, C, B}, {A, C, B, A, B, C}, {A, C, B, A, C, B}, {A, C, B, C, A, B}, {A, C, B, C, B, A} = 10 permutations.
That is it, unless I made mistake somewhere!.