An X3 Shock modifier does three times the listed damage of the weapon. However, this is dependant on the enemy you're fighting. If you're fighting something which has large shields, it may do more damage. If you're fighting something which doesn't have shields it may do less damage. If you're fighting something which is Shock resistant you may do even less damage. Keep in mind that Shock weapons are best used against enemies with very large shields (Guardians, etc) and if your enemy uses a specific attack it may very well be immune to that type of attack itself.