Source: billandted3.com

Behold:

Exhibit A:

Though it isn't enough to confirm a third film, there have been indications to a possible sequel, through a bunch of news bits over the past couple of years:

From Wikipedia:

In 2010, Reeves indicated that Matheson and Solomon were working on a script for a third film.

Winter has stated that the role of Rufus (previously played by the late George Carlin) would not be recast.

In April 2011, Reeves stated that a third film was close to happening. On April 24, 2011, Winter tweeted that the script for the third installment had been finished

So... yeah. The scripts finished, it's just a case of the project being greenlit and such.