Understanding LTI systems graphically
Suppose that you could write $x_.$ as $x_.(t) = a_1x_1(t-n_1)+a_2x_1(t-n_2)$, then by LTI hypothesis, you can derive $y_.(t) = a_1y_1(t-n_1)+a_2y_1(t-n_2)$ with very little computation.
You can get a first insight to that method  by looking at the shape of $x_2$ which is self-evident, and a little more imagination gives you the structure for  $x_3$.
I find that this exercise is very pedagogic in the progression of difficulties. Some other exercises of the book might have the same gradation, don't hesitate to look back before jumping into computations at first.