3 Divisible Subsets

Problem Today Bhrigudov was getting bored in his class so he flipped to the last page of his textbook and founded a lot of random positive integral numbers printed on it. So, to get rid of boredom he thought, how many three element subsets can I make from these numbers such that sum of the elements of the subset is divisible by 3. As there can be too many such subsets, he turn to you for help.