We define two variables first and second as the first two of the increasing subsequence of length 3, and first2 and second2 as the "backup", we replace first2 and second2 for first and second at the right time.
Here comes an example. If the array is [1, 2, -3, -2, -1], firstly, first = 1, and second = 2, then comes first2 = -3, second2 = -2. After comparing, we replace first2 = -3, second2 = -2 for first = 1, second = 2, because it is more likely to reach the goal.