There are some rumours going around that she will, but nothing has been confirmed.
I hope she is. ^_^
So far the character has not been added to the possible cast list. Only The Riddler has. But keep checking IMDB.com for the cast list (search Batman 3) In short, no. The villains for the third installment are Bane and Catwoman. Also with there almost certainly being no joker in a Batman movie for a VERY long time, it's possible that we will never see Harley in a movie
no (i wish that would have been awesome!) but most likely she'll be in the next movie
Unfortunatley, no. The next Batman is going to be directed by Tim Burton. The villains will be the Riddler and Catwoman as played by Burton's two favorite actors. <- This is wrong, the next movie will be directed by Christopher Nolan. The Riddler is not going to be in it but Catwoman will be. The pics on google are incorrect and are just fan art. Can't wait!!
It has been confirmed Tom Hardy is playing Bane and Anne Hathway is playing Catwoman.
The name of the next Batman movie is called "The Dark Knight Rises". This has been confirmed by Christopher Nolan.
Bane
July 20, 2012
It is Bane and Catwoman.
The star of the next Batman movie has not yet been determined. The movie has not even begun production yet. However, the film is anticipated to be a reboot of the franchise with an all-new cast.
Bane (played by Tom Hardy)
the dark knight rises i believe
The Dark Knight Rises (July 20, 2012)