Nighthawk Scavenger counts card types, not cards. If the only card in your opponents' graveyards is a single artifact creature card, Nighthawk Scavenger will be 3/3. If the only cards in those graveyards are ten artifact cards and ten creature cards, Nighthawk Scavenger will still be 3/3.

The ability that defines Nighthawk Scavenger's power applies in all zones, not just the battlefield.

Card types that can appear on cards in a graveyard include artifact, creature, enchantment, instant, land, planeswalker, and sorcery. Legendary, basic, and snow are supertypes, not card types; Vampire and Rogue are subtypes, not card types.