Answer
spring 3, followed by a tie of springs 1 and 2
Work Step by Step
Potential every is stored energy due to position; and the farther you stretch a spring the more energy you store. The springs with the most potential energy at $t=0$ will be the ones with the farthest displacement, $x$. Looking at the graph, we can see that at $t=0$; spring 3 has the largest displacement, and therefore the most potential energy, followed by a tie of springs 1 and 2.
