Below is a rough translation of this article from the Bulgarian site 24chasa:
"Expendables 3" will show as Sofia Romanian city. Within a few days the team of Sylvester Stallone, who has two months in the country to film the third part of the action movie "The Expendables" will close the city center. The area around the hotel "Sheraton" and Party House - is now the second building of the National Assembly, the team has a plan to shoot footage of many bodies and explosions.
Actually, the action will be filmed in the city center, but the script is called the Romanian city. It is very likely that this is Bucharest, capital of Romania.
Actors to plan to shoot this scene next week. In recent days, however, their program are often amended at the last minute, said people in the cinema center "Boyana". There is talk that the cause is a mild illness Sylvester Stallone. Gibson, however, is on the set. Yesterday, the American actor has shot important scenes action from morning to evening in Boyana.
By the end of this week, the team has to move and the government residence "Lozenets". It is also included in the program of Stallone, he even has official permission to film inside and in the courtyard of the military club."
Here's another article talking about the filming of this action scene with a photo of the location. It looks interesting, we haven't really seen The Expendables in an urban setting like that before:
A story action in these frames are not developed in Bulgaria and Romania."
With Mel Gibson and Robert Davi now on set and this action scene happening in the city hopefully some great photos will turn up next week!