things change over time though and i believe that they already confirmed that Chicago is the final battle. They might have a different/smaller battle set in Detroit though, never know
It could be that they are filming scenes for the final battle in Detroit, but if the movie, it will be Chicago. If my memory serves, the scenes in Mission City in the first film were filmed in Detroit.
Nope, it was LA...but the scenes in the building Sam runs into with the Cube were filmed Detroit I think.
True, scenes for the Mission City battle were filmed in Detroit, but not the entire battle. They shot parts of that battle in L.A., I believe. So... it could be possible that parts of the TF3 final battle will be filmed in Detroit, or whatever's being filmed in Detroit is entirely something else. Only time will tell. But it does sound like the battle in Chicago IS the final battle.
I see no reason to either believe or disbelieve them on this. Final battle could even be on Cybertron or outer space. There's no way of knowing for sure till it actually goes on release.