To cast Lullmage's Domination, choose a value of X, choose a creature with the appropriate mana value, then determine the spell's cost, subtracting {3} if appropriate. This can't reduce the {U}{U}{U} in the spell's cost. For example, if you wish to target a creature with mana value 2 whose controller has eight cards in their graveyard, you'll pay {U}{U}{U}.