What would benefit the mercenary most? Damage? Life Leech? Status effects? When you find out what would help him do a better job for you, then you can decide what to put inside the three socket one. 3 Ohm's for Damage, or 3x40/15's for damage and speed. Or maybe you want some Amn's for Life Leech, or a combination of those. Or you could be nuts and put a Cham, 40/15, and Amn for a weird combination. But yeah, find out what would improve your mercenary for you the most, then work to get it and put it in the 3 socketed one.