Puzzle of the Week #522 - Geometric Sequence

The sum of an geometric sequence with n terms, the first of which is 1, is 127:

 1 + r + r^2 … + r^(n-1) + r^n = 127

 A subsequence that includes only every other term, but still starts at 1 and finishes at r^n totals 85:

 1 + r^2 + r^4 … + r^(n-2) + r^n = 85

 What is the sum of a subsequence that includes only every third term, but still starts at 1 and finishes at r^n:

 1 + r^3 + r^6 … + r^(n-3) + r^n = S