I inputted these three Zacian into Pokémon Showdown and their total Attack stat was as follows:
Zacian 1: 176 Base Atk
Zacian 2: 182 Base Atk
Zacian 3: 172 Base Atk
If you quickly glance at the sets, you will see that Zacian 2 & 3 have the same nature, but Zacian 2 has higher EVs, automatically making Zacian 2 higher than 3. The Adamant nature for Zacian 1 shakes things up a bit, but ultimately Zacian 2, a much more standard set in my opinion, is the hardest-hitting Zacian of the three. Hope I helped!