No announcement on an official third "Zoo Tycoon" game has been made by developer Frontier Developments or publisher Microsoft Studios. A third game in the series was released exclusively for Xbox 360 and XBox One, but this was not the official "Zoo Tycoon 3".
The third major "Zoo Tycoon" game was released in 2013, but the developers did not choose to call it "Zoo Tycoon 3." This game is available exclusively for the Xbox 360 and Xbox One. Since the developers chose to go back to the original name, it is possible the next game in the series may also not be called "Zoo Tycoon 3." Any news about new "Zoo Tycoon" games in development should be posted on Microsoft's official page for the game or in the "News" section of Frontier Development's website.