EV adds to your stat, not multiplying like nature.
So if your pokemon has lower HP than other two(ex.Shuckle),
give 252EVs on HP and rest on Def and Sp.Def
So give your EV on the defensive(HP/Def/Sp.Def) stat which it lacks on
But there are some exceptions.
for example, for eviolite dusclops or dusknoir that has pain split,
you should give 252def and 252sp.def, since painsplit is better move if you have lower HP
and for blissey with counter, it is better to give 252 on def(since it can be OHKO)
and 252 on HP(for counter)
The stat is calculated by
when it reaches lv.100
so it is best to give your pokemon's EV that is multiple of 4, or it can be wasted