According to Ann's blog she is planning to write another one in the Traveling Pants series because she has been thinking about the characters a lot lately. That is something I find fascinating about writers--they spend so much time with their characters that the characters can live on after a book or series is finished. I would think it would be hard to let them go. Anyhow, I have not yet read 3 Willows but so far reviews have been very positive.
****Interesting side note: Ann Brashares grew up in Washington DC, and attended the Sidwell Friends School. Sidwell has been in the news of late because the President-elects daughters are currently enrolled. Other notable alumni include Bill Nye (yeah the Science Guy) and Charlie Gibson.