d = discount on third shirt
c = total cost
Sales tax of 7.1% equates to 1.071
c = (Cost of 1st Shirt + Cost of 2nd Shirt + Cost of Third Shirt) * Sales Tax
c = (49.96 + 49.96 + (49.96 - d)) * 1.071
Since you didn't specify the discount on the third shirt, this is the best I can do for you.
The mean of the cost of the shirts would be c/3