You're not supposed to pass cars that are on your left in france, from which results that the slowest vehicles should always be in the right lane and fast ones in the left lane.
So be careful with that because someone going from the middle lane to the right lane will not always check his mirror to see if someone's coming faster than him from behind on the right lane (they're not supposed to).
For this very reason it can make it incomfortable to drive on the right lane when people are being jammed on the middle lane, especially if you get stuck behind a very slow vehicle on the right lane and can't move back to the middle lane because it's jammed.