3-partition problem: https://en.wikipedia.org/wiki/3-partition_problem
The problem is to decide whether a given multiset of integers can be partitioned into triplets that all have the same sum.
Example: The set S = { 20, 23, 25, 30, 49, 45, 27, 30, 30, 40, 22, 19 } can be partitioned into the four sets: { 20, 25, 45 }, { 23, 27, 40 }, { 49, 22, 19 } , { 30, 30, 30}, each of which sum to T = 90.
Another example; the set S = {1, 2, 5, 6, 7, 9} can be partitioned into the two sets {1, 5, 9}, {2, 6, 7} each of which sum to T = 15.