CN-TST 2015 Problem 11

Set $S$ to be a subset of size $68$ of $\{1,2,...,2015\}$ . Prove that there exist $3$ pairwise disjoint, non-empty subsets $A,B,C$ such that $|A|=|B|=|C|$ and $\sum_{a\in A}a=\sum_{b\in B}b=\sum_{c\in C}c$