You need 3 champs all of the attack type, then just simply put gloves, boots and chest plates on them that all have attack percentage as that price of equipments primary stat. That being the stat that changes every time the equipment is upgreaded.
You'd need to equip one Attack type champ with Gauntlets, Chestplate, and Boots with Attack% as their primary stats- just like the example I've shown. Hope this clears it up.