I defeated the Cathedral of Light Capstone Dungeon and even have credit on my account for beating it and have a lvl 50 Druid but Tier 3 did not unlock.
same problem here. I have completed the campaign and the final dungeon (both tier2) but tier 3 nightmare did not unlock.
i play as a druid too.
edit:
i cleared the final dungeon again (standing in the middle in the circle) and unlocked tier3
It’s a bug to do with Necro summons and Druids.
If you can run a non summon build for that boss it should work.
Lets hope Blizzard fixes it soon, but I happened apon it 3-4 days ago now.
You are better off doing without summons or grinding for more power to nuke the boss (super inefficient)
Also stay in the middle of the room, heard some say the ring of the room can cause it.
Thanks all, I didn’t do a summons and went with a werebear build and it passed as soon as I killed him.
Possible solution: See if the World Tier Statue in Kyovashad has WT3 unlocked. It worked for me after killing Curator. WT3 was locked in game menu but it was unlocked via the statue.
Edit: This was on Necromancer with no minions.
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Indeed, I finished as an 11 summon necro and could not select from character screen but the statue unlocked it/ Ty for that =D
Just did it with Rogue (43), WT3 still locked. 
I guess I’ll do it over and over and over…