For the Germans: they had a chance to defeat France at the Marne, and a second chance in 1917, when the French army mutinied. An all-out attack could have overrun the French lines. The biggest mistake took place decades earlier, when Chancellor Caprivi let Germany's secret alliance with Russia lapse and the Russians allied with France instead.
Austria: best if it could have isolated Serbia in 1914. If the Austrians could have shown that the Serbian government was tied to the assassination, it would have been more difficult for the French and Russians to support Serbia. The British would have pushed them strongly to back off. Also, Italy was dithering, angling for the best deal. It would have been cheaper to give Italy some territory in the Tyrol or Dalmatia, or to acquiesce in a takeover of Albania, to get it on the Central Powers' side.
Turkey: play the pan-Turkish card in Central Asia earlier. Provide Nejd with an incentive (control of the holy cities?) to join the fight against the Hejaz revolt. Turkey didn't have to get involved in WWI at all, and ultimately it might have been better off staying neutral.