is this normal for French shows?
A three-year wait between seasons?..
Yes.
The French will do a show with 6-8 episodes and then stop for what seems like FOREVER. The actors will go on to do other TV shows or movies, and then return to the series... not necessarily picking up right where they left off in the storyline, but instead at some later point (as if it were a mini-series sequel). Luckily, (in most cases) they tend to end a series' season with some pretty good resolution rather than with the kinds of extended story arcs and cliffhangers we might see in US/UK TV series.
Knowing this, I've put off finishing Season 1 of Les Revenants because I was hoping that this past year or so would be enough to have Season 2 produced. I was also hoping that the series would be produced at a faster pace since it has received such international acclaim.