I have not tried it, but i think 2TB was the limit(but it should be higher).
Most of the utilities to do this seem to be for Windows only. Do you have a windows computer or windows under Boot Camp?
I am not sure if you can use a 3TB partition with Xbox360, it may have to be under 2TB. I do not have a 360 to test with(hell my biggest drives are all 2TB as well.)
Most people seem to be using this, but its windows only. http://www.ridgecrop.demon.co.uk/index.htm?guiformat.htm