Gain control of target creature with mana value 3 or less until end of turn. Untap that creature. It gains haste until end of turn.
Claim the Firstborn can target any creature with mana value 3 or less, even one that's untapped or one you already control.
Gaining control of a creature doesn't cause you to gain control of any Auras or Equipment attached to it.
If a creature on the battlefield has {X} in its mana cost, X is considered to be 0.