3. (C) Solution 1 - Let $x$ and $y$ be the lengths of the long and short tracks, respectively.

In one week, he runs $6(x+2 y)$ and in the other $7(x+y)$. Since in each week he runs the same 5000 meters, we have: $6(x+2 y)=7(x+y)$.

It follows that $6 x+12 y=7 x+7 y$, and therefore, $5 y=x$. Thus, the length of the long track is five times that of the short track.
2 - In the week that Joãozinho trained for seven days, he ran one more long track and five fewer short tracks than the week he trained for only six days. Since the distance run was the same in both weeks, we conclude that the length of the long track is equal to the length of five short tracks.