For a film that is still technically the highest-grossing film of all time, Avatar has a surprisingly small pop-cultural footprint, which really sucks for the people who put in the effort of learning Na’vi only to be asked if they speak High Valyrian at fantasy conventions. It sucks somewhat less for James Cameron, for whom the Avatar business is recession-proof for reasons presumably related to the “highest-grossing film of all time” thing.
So it’s not really that big of a deal that Cameron is pushing back Avatar 2’s scheduled release date of Christmas 2016 to Christmas 2017, saying his plan of making three Avatar sequels at once may have been just a tad ambitious. (Making the cinematic equivalent of the brown note takes time, apparently.) Cameron also says that once the sequels have been completed, one will come out each subsequent year thereafter (so in 2017, 2018, and 2019), ensuring that we will have put the whole thing behind us by 2021.