1 eq/L neutralises 1 eq/L (eq = equivalent)
A 3N solution of Mg2+ (3eq/L) will neutralise a 3N solution of OH-.
The ratio of charge is 2:1 thus 2 OH- ions neutralised by 1 Mg2+ ion.
So the molar concentraions would be [OH-] = 3M and [Mg2+] = 1.5M per unit volume.
Here the molarity of Mg2+ is 0.5(Normality).
Using this principle, the normality will always be greater than or equal to the molarity value since half a charge does not exist.
Another perspective: In a xM solution of an ion with charge = y, normality = xy.
Eg. 0.15M of Al(OH)3 = 0.15mol/L. Since charge of Al ion = 3+.Normality = 3(0.15) = 0.45 eq/L = 0.45N