The answer is 6.0 moles of HCl is required to neutralize 3.0 moles of Mg(OH)2.
The reason is because of the mole to mole ratio between HCl and Mg(OH)2.
Balanced equation: Mg(OH)2 + 2HCl = MgCl2 + 2H2O
If we look at the balanced equation between these two molecules, we can see that for every one mole of Mg(OH)2, two miles of HCl is required.
Therefore the answer is 6.0 moles.