so I box 3 mages. I love the insane dps and how fast i down bosses. i have a few issues.

i just did my first heroic while 3 boxing. heroic stonecore. on the first boss i we kept wiping....i dont really see how it was my fault but the tank was saying it was impossible for me to do some encounters......if someone could enlighten me on if 3 boxing is a dumb idea?