This site has posted video of EX3 fliming at the Naval Station in Varna. It isn't the best quality, but you can see the huge scope of the production.
Also, another article had this to say, translated poorly, of course:
World famous actors Sylvester Stallone and Harrison Ford walked into our Military Academy (MMA). But not as patients, but to capture successive scenes from the movie "The Expendables 3". It reported "Trud" happy witnesses ended up by chance the exciting photos.
Around 11 am on Monday the rear entrance of the MMA had already acquired the American type. The film crew had set shady palm trees within a radius of a few meters, simulating overseas environment and the porch of the local hospital. "Suddenly appeared shiny black limousine as a typical US ambulance with blue inscriptions taken from the cinema center in" Boyana ", said patients. Minutes later, glowing and Hollywood stars - Sylvester Stallone and Harrison Ford.
In one of the short breaks when cast down gene. Stoyan Tonev, head of the Military Medical Academy. They shook hands with Stallone and Stallone thanked him for "field" with the words "Tenko Yu, General."
Details only read in the paper edition of "Trud"