Cameron plans to film the highly-anticipated sequels at one time and then release the three films over consecutive years.
The first had been due in late 2016, but he said the writing process had proved to be "very involved" and that the initial target date was probably too ambitious.
"There's a layer of complexity in getting the story to work as a saga across three films that you don't get when you're making a stand-alone film," he said in Wellington, New Zealand, where he was helping promote the local film industry with other directors including Peter Jackson.
Cameron said the goal was to have the scripts for the three movies completed by the end of this month. He's leading a team that's writing the screenplays.
"We're writing three simultaneously. And we've done that so that everything tracks throughout the three films. We're not just going to do one and then make up another one and another one after that," he said. "And parallel with that, we're doing all the design. So we've designed all the creatures and the environments."
Cameron said he thought it was important that each film linked forward to the next one in a satisfying way but also came to a resolution so that the audience wasn't left hanging.
Released in 2009, "Avatar" became the highest-grossing film in history, with a box office take of nearly $2.8 billion. It won three Academy Awards.
Cameron shot the original movie in New Zealand and is planning to shoot the sequels there.
Producer Jon Landau, who is working with Cameron on the movies, said the team is taking greater security precautions following the recent hack against Sony: "I will only say 'yes' but will not tell you how," he joked.
The films will be made by Lightstorm Entertainment and Twentieth Century Fox.