Update (23/02): it looks like Oh My Disney made a mistake, which has been fixed since we posted the news: Dan Scanlon won't direct Cars 3 after all. Which means we still don't know who is directing a film that's supposed to be released next year. We wouldn't be surprised if the movie gets pushed back.
Original post (22/02): It's been rumoured for quite some time, but this post on Oh My Disney seems to make it official: Dan Scanlon (Monsters University) will direct Cars 3 for Pixar.
Dan Scanlon seems like a natural choice to helm the third installment in the Cars franchise, as he was a story artist on the first one - he also co-directed a short set in the Cars universe, the spooky/funny Mater and the Ghostlight.
Scanlon has already shown with MU he can take an existing franchise and create something fresh from it; hopefully that's what he's doing with this new Cars film, as the second one wasn't a big hit critically, to say the least.
Cars 3 is scheduled for a June 2017 release, but we know nothing about the plot of the movie yet.