Error with mission 11
I've played it on easy, and I quickly took out the dude standing at the outside before the people even got out of the car. The two men that walked out, I shot the second one before he go inside the building, so they were technically seperate, and I shot the one that heads out first in the first open window area. That is where the second man to exit the car would typically stand and pause for a moment, as the first walked up the steps.
Perhaps the fact sometimes seeing a dead body doesn't neccessarily alert someone would be the problem? Such as the tower level, where I killed one on the ground and the other walks pretty darn close to it without noticing. Perhaps you didn't expect anyone to shoot that quickly without thinking.
Anyways, I had managed to kill all 3 without any level advancement. The door guard and the first to come out of the car were dead on the doorstep, and the final guy died the moment he appeared in the first window area. I hope the information helps you at all, because I personally feel very pulled into the game and saw this really as the only flaw.