As long as you can find a triplet [num1, num2, num3] in a sorted array which satisfies num1 + num2 + num3 < target, their indexes

in the original array can always be expressed in ascending order. i < j < k.

e.g, in the original array their order could be [num2, num1, num3], [num3, num1, num2] etc.

So, i is the index of the first appeared number of num1, num2 and num3 in the original array, j is the second and k is the last.

Actually, indexes of any 3 numbers in an array can be expressed in ascending order.

Just sort it.