Time complexity about Maximum subarray

I recently came across a function called the strawman algorithm which the pseudo code looks like this:

StrawmanSubarray(A):   Initialize bestsum = A[0]        For left=0 to n-1:         For right = left to n-1:            Compute sum A[left] + . . . + A[right]            If sum > bestsum: bestsum = sum 

The time complexity is Θ (n^3), and I don’t quite get where is the 3rd n comes from to get Θ (n^3)?