May 31 (Reuters) - Three of the hedge fund industry's most closely followed managers reported mixed returns in May, underscoring just how tough it has become to pick winners.

David Einhorn's Greenlight Capital dipped 1.9 percent in May, leaving the fund up 1.1 percent for the year so far.

Daniel Loeb's Third Point Partners fund gained 1.7 percent in May and is up 1.5 percent for the year. Barry Rosenstein's Jana Partners fund gained 2.4 percent in May, but is nursing a 4.7 percent loss for the year.

The funds do not release their numbers publicly but a source who invests with them shared the data with Reuters.

(Reporting by Svea Herbst-Bayliss in Boston and Arathy S Nair in Bengaluru; Editing by Leslie Adler)