Sometimes it may help to construct one of those tree diagrams you've seen.
\(\displaystyle \L\\s=x(t,u), \;\ t=y(s,u), \;\ u=z(s,t)\)
\(\displaystyle \L\\G(x(t,u), y(s,u), z(s,t))\)
\(\displaystyle \L\\\frac{\partial{du}}{\partial{ds}}=\frac{\partial{du}}{\partial{dz}}\cdot\frac{\partial{dz}}{\partial{ds}}\)
\(\displaystyle \L\\\frac{\partial{dt}}{\partial{du}}=\frac{\partial{dt}}{\partial{dy}}\cdot\frac{\partial{dy}}{\partial{du}}\)
\(\displaystyle \L\\\frac{\partial{ds}}{\partial{dt}}=\frac{\partial{ds}}{\partial{dx}}\cdot\frac{\partial{dx}}{\partial{dt}}\)