A rumor we have been tracking has been confirmed. The third theater for Soarin' was originally reported by Jim Hill back in March of 2012. The rationale was linked to capacity increases that were necessary for the Next Gen project.
Yesterday, Disney filed a Notice of Commencement for Project 87 to "Add facility to existing attraction". Credit goes to WDWMagic.com user danlb_2000 for identifying the Notice of Commencement and peter11435 for identifying the significance of Project 87. Each Soarin' theater has 87 seats, indicating that this Notice of Commencement is indeed for the third theater.
The expected time frame is contingent on the Frozen attraction for Norway. The current goal is to have the Frozen attraction open in December 2015. The expectation is that Soarin' would close shortly thereafter (likely after the holidays) and then re-open 6 months later with a third theater, a new Soarin' Over the World film, and upgraded Digital projectors. WDWMagic.com user Alektronic has reported the 6 months time frame.