1. If we add the sides together we get 7n=7070 so n=1010 which leads to a negative side length for one of the sides, because 3n-13n-1=-10n-1, so there seems to be an error in the question.
I suspect the three sides are n, 3n-1, 3n+1, in which case the sides are 1010, 3029, 3031.
2. Let the shortest side be length S. Then 2S+6 and S+21 are the lengths of the other sides, so 91=S+2S+6+S+21=4S+27, 4S=64, S=16 and the lengths are 16, 38, 37.