Let the radius of R's track be $r$ and let the radius of the first semicircle of P's track be $p$; then the radius of the second semicircle of this track is $r-p$. The total length of P's track is $\pi p+\pi (r-p)=\pi r$, the same as the length of R's track. By a similar argument, the length of Q's track is also $\pi r$, so all three runners finish at the same time.