The three elites(/champions?) you were talking about are not a boss, but just some tough mobs to fight on the way to the last boss. Pulling three at a time is usually a wipe...
You should be able to pull just one at a time, or at most two. Wait for the walking ones to be close, and pull them separately, then try just pulling the nearest one, etc.
Don't sweat it. We all make mistakes. You definitely need a thick skin to tank. People will complain no matter what.