I'm not sure if there will even be a third Deus Ex considering that Eidos Interactive closed down Ion Storm (the developer of Deus Ex) over a year ago. Most probably, it will be up to Eidos to decide who to continue the development of this series.
And if there ever becomes one - I hope it is like the first installment and not like the second. First was good RPG/FPS hybrid - second was just another run-of-the-mill FPS in my view.