...That's "at once" as in "at the same time," not at once as in "Fetch me my scepter! At once!"
Yes, James Cameron seems to be planning to shoot three enormously production-heavy films all at one time. In an interview with Showbiz 411, Sigourney Weaver said that she is gearing up to shoot Avatar 2, 3, and 4 after a brief run in a stage production.
Weaver has no idea how long the shooting will take, as Cameron seems to have kept most of the details of the films under wraps--even to his own actors. Cameron has stated recently, however, that he has been spending the better part of the last two years on developing software to make the whole process easier.
Now he may well spend the better part of another year or so filming the projects. What it means is that, while we will have to wait at least 4 years from Avatar's 2009 debut for the first Avatar sequel, the second, third, and fourth films may only have a year between releases.