jmc

Let the side length of each square $S_k$ be $s_k$. Then, from the diagram, we can line up the top horizontal lengths of $S_1$, $S_2$, and $S_3$ to cover the top side of the large rectangle, so $s_1+s_2+s_3=3322$. Similarly, the short side of $R_2$ will be $s_1-s_2$, and lining this up with the left side of $S_3$ to cover the vertical side of the large rectangle gives $s_1-s_2+s_3=2020$. We subtract the second equation from the first to obtain $2s_2=1302$, and thus $s_2=\boxed{651}$.