There are no charms for ranged damage. The melee damage charm only works in melee weapons. I assume, but have not tested, that a magic damage charm would add to the magic damage done by bows or melee weapons.
There isn't really an obvious third choice for weapons with 3 enchantment slots; I went with Poison for my warrior and Butcher for my ranger.
I never see a Magic Damage Charm

... only Increased Magic Damage via Enchantments .
Why not Fire Curse, Life Leech or Mana Leech as third choice on a weapon ?