Do I think there should be a Portal 3? No. Portal 2 wraps things up nicely. Chell is free to go, GLaDOS is alone again with her robots to test with.
Could it be done? Maybe. GLaDOS might ask Chell to do something for her which she can't do herself or via robots, and given her history, Chell could easily be the best candidate for the job.
Would I like to see another game with Chell in it? Sure. Maybe she could get added to the G-Man's payroll along with Gordon Freeman. Have two single-player campaigns. One for Chell and one for Gordon, with the two occasionally crossing paths, but otherwise having their own independent missions.