Im usually fine with imbalance, but when it gets to these extremes it just destroyes variety of other suboptimal mercs. A3 fire merc (lvl 98) does a ovewhelming 450 fire damage. he cant even kill baal wave 5 in normal with that. the other a3 are even worse damage wise. a3 cold is always inferior compared to a2 holy freeze. Worse AI in terms of attack frequency, worse damage, worse stats. A3 mercs simply are missing Synergies and masteries. the merc synergies were probably overlooked after synergies were introduced.
I took these numbers from the other merc thread from Rondel:
Act 1 Fire max damage @ 900 (dps @250)
Act 2 might max damage @ 900 (no might)/1500 might (dps @2K)
Act 3 fire max damage @ 200 fireball (dps @400)
Act 5 max @ 1400 (dps @500)
he did not go into detail what gear they used, for the barb it was a 2h grief. the numbers are just beyond any imbalance one could take. you could easily double a1 & a5’s damage and they would still only have 25% and 50% of what a2 does damage wise. There is a reason why you only see a2 mercs.