(In reply to Possible Solution
I found (A,B,C) = (5,15,80) is an answer with progressions 100, 90, 80 or 81. This corresponds to d=-10. The sum A+B+C is 120 in this case.
Similarly, d=-30 yields (A,B,C) = (45, 145, 840) for a sum of 1030 and d=-40 yields (A,B,C) = (80, 780, 1520) for a sum of 2380.
Also, you have a typo C=d(d+1) should be C=d(d+2)