First of all, there is a difference between reducing received damage and reducing enemy's damage . Nord passive and ice fortress reduce the damage dealt to you. Heroic slash applies minor maim debuff that reduces the damage dealt by your target. So they do not affect each other in any way.
Ice fortress and nord passive do stack but they are multiplicative and not additive. So you won't get 14% reduction. The actuall number will be lower (around 10-11%). Nord passive normally diminishes itself to 2-3% due to the way damage mitigation formula works.