The main issue with Act 3 Mercs is the following.
Little damage (compared to others).
Little survivability (compared to others).
Little stats (compared to others).
Little Damage
As far as damage goes, I believe just adding masteries solves this problem a bit. It can still use different spells for some of the class archtypes, but a more immediate quicker fix that doesn’t need much testing is just adding masteries on them (cold, light, fire).
Little Survivability
I will pair this hand in hand with a little Damage. In order for you to increase the damage of your merc, and made it viable, you need to sacrifice high survivability gear for high defense gear. That’s not the same as act 2 or act 5 mercs, where you can stack them with high suvivability gear thats also really good for them offensively.
At level 86, the merc has 63 resist all, which is actually quite bad because you need to sacrifice potentially damage that he doesn’t really have to begin with in order to build up the resists.
I think that the Act 3 Merc alone should natively have high base resistances and they should cap at 85 instead of 75. This should also be achievable to have 85 resist all at like level 85 or even level 90. It should be a strength of theirs where their resist is just naturally high. They still will suffer defensively from physical, but against magic they should excel and that should be one of the strengths.
Little Stats
At level 85, the merc had like 136 strength.
I tried to beef up the damage of my merc by providing him with a Eth Grypphons, Eth Ormus with +3 charged bolt, and Spirit Sword. I was shocked that I couldn’t actually equip a Spirit Shield.
It’s kind of like the most basic shield that you would use for a merc, and it seems odd that even at level 99 he wouldn’t be able to equip it without socketing like Fal runes for stength, again sacrificing potential damage/survivability.
All in all, the following easy 3 things should drastically improve the act 3 merc, not so much where it becomes meta because act 2 is still always best, but at least to the point where its not just a waste of time entirely.
1. Increase Base Str by like 30-40.
2. Increase Base Resistances by 15 and allow it to overcap to 85 resist all instead of 75.
3. Added Fire/Cold/Lightning Mastery to the skill Trees.
These 3 additions (which can hopefully come in an update during this ladder) would be GREATLY appreciated. It also wouldn’t do anything to the viability of the merc because act 2 and 5 are always going to be better options, but it will at least be a quick good change that will help significantly.