The three rings in the WW/Rend build are CoE (equipped), BoM (equipped) and ORotZ (cubed)…
In the season where we had a 4th Cube slot, we could Cube both the ORotZ and BoM, and wear Focus + Restraint, because maintaining the F+R bonuses was better than CoE. That’s no longer available, so you need the three core rings, because if you Cube ORotZ and wear F+R you’re missing out on the massive protection from BoM.