3 investing lessons from Einhorn's candid letter

David Einhorn
Adam Jeffery | CNBC

David Einhorn, the legendary hedge fund manager whose stock picks move markets, had a tough first half of 2015. The third quarter was even worse.

His fund, Greenlight Capital, was down 14.3 percent in the third quarter, bringing its returns for the year to a negative 17.1 percent, according to an investor letter sent to clients Wednesday.

Einhorn's letters are revered by investors for their transparency, rational analysis and nuggets of wisdom. The latest missive did not disappoint as the fund's poor performance caused Einhorn to candidly examine how he invests.

Here are three key investing lessons for the regular guy from the hedge fund manager's letter.