It is finally confirmed and by the second man in command, Statham. In a recent interview published on Shock Ya Movies, when asked about his projects beside F&F7, Statham responded: "I’m going to do ‘The Expendables 3.’ We start on August 20 in Bulgaria".
So EX3 is up next for Statham, with F&F7 shooting sometime soon afterward, according to an interview given to MTV.
So here we have it. Arnold was right, production starts in August in LA early August, then moves to Bulgaria. Hopefully Arnold follows the crew to Bulgaria and doesn't stop at LA or we'll have another cameo of the man, crossing fingers to see him with a bigger role than what he had in EX1-2.