Does this work with one of the iron wolf A3 mercs magics?

No

if you want to use lawbringer to full efect you'll need an act 5 merc

A3 mercs usually cast, but dont hit with their sword. They will not cast c2c on hit spells

act 3 mercs will only hit with their sword when they're already in melee range. Then they'll have a very high AI roll to run away instead of staying in place, and if that fails they'll have an AI roll to either attack or cast a spell- 10/100 for fire/light mercs, 10/310 for cold mercs with armor active, 10/1310 for cold mercs with armor inactive. Then only if that attack actually connects will it roll a 20% Chance To Cast Level 15 Decrepify On Striking

Thanks for the info. Looks like no need for act 3 merc lol.