Let's assume that A, B and C are X, Y and Z years old respectively this year.

According to the meaning of the question, there are

x+y+z = 1 13 ………( 1)

2( 17-x+y)= 17-x+z……(2)

From (2):

x-2y+z= 17…………(3)

(1)-(3), so:

3y=96

Y=32 years old