So I was bored so I typed "Portal 3" into Google and one of the top results was a link to a Wikianswers page that had the question "What will the plot of Portal 3 be?" So out of boredom I clicked on it and heres the answer it gave.

Wheatley is back and wants revenge on GLaDOS for calling him a "moron" So you must play as Atlas and P-Body (the co-op robots of Portal 2) to save GLaDOS and kill Wheatley once and for all

(I really hope its right about the killing part :D and I wouldve made this an article but it said it was too short :P)