What is the Might and Magic: Heroes 6 Damage Computation?

There is a fan created Heroes VI manual that contains all the gritty details of the damage calculation. This is not an official manual, so no guarantee it is correct.

Essentially there are two calculations:

Raw Damage = Creature Damage * Stack Size * Attack Modifier

Creature Damage and Stack Size are what you think they are. The Attack Modifier is a separate calculation based upon the hero's Might or Magic Attack value, depending upon the type of damage the creature inflicts. The Raw Damage is calculated as a range, due to the Creature Damage not being a fixed number.

The second calculation brings in the defense of the stack being attacked:

Final Damage = Raw Damage * (100% - Resistance %) * Misc Modifiers

The Misc Modifiers are bonuses given by spells, skills or artifacts (i.e. friendly creatures' ranged attacks deal 8% more damage). Resistance % is calculated in a similar manner to the Attack Modifier using the defending stack's and hero's Might Defense or Magic Defense.

Final Damage is also a range (as Raw Damage was) and the calculation then picks a random integer out of that range for the actual damage caused by the attack.

As to your comment question regarding +3 Might Defense, that does mean +3 to the the Might Defense value, which is turned into a percentage during the Resistance % calculation.