The weapon has some sort of elemental aspect that increases damage. Anyone damaged by an elemental weapon may take additional damage at the end of the shot. Unless the target has some extra resistance to the damage type (such as fire or acid resistance for those attacks), he takes 3 additional wound points.
Someone immune to melee weapon damage ignores any Wound Points from the weapon itself, but if the attack overcame Toughness/Armor and would have inflicted damage except for the resistance, the target still take the three extra elemental wound points.
A target immune both to the elemental damage and the regular damage takes no damage at all.