Napoleon's three mistakes were using his inferior French navy to battle British naval forces, waging war against Russia and resuming power after he was exiled. These three mistakes greatly damaged his reputation.
Napoleon made the first major mistake in the early 1800s when he attempted to gain control of the sea by attacking the British navy. Napoleon's navy was destroyed, along with the Spanish fleets that aided his cause, and the British retained control of the seas.
Napoleon then led a campaign against Russia in 1812, but the Russians forced the French army to venture deeper into Russian territory by making retreats. The Russians destroyed their camps, and Napoleon's men became trapped in the frigid Russian terrain while facing an onslaught from the Russian army. Napoleon began his campaign with 600,000 men, but only 100,000 left the country.
Napoleon resumed governance of France shortly after being exiled in 1815, but old frustrations lingered in his government. He suffered a massive defeat at Waterloo by the British in 1815, which led to his ultimate downfall.