Hello Sarah, you shouldn't be taking the total length and subtracting 2y+3 and y-1. From reading the question, you can find out that B is the longest as it is longer than A and C. Hence, to find 3 times the length of B, you take the total and add the difference between A and B, and the difference between B and C.
Hope this helped, do comment if you have any other questions. :)