Blizzard has added a new “Triple Threat” title in Patch 11.1.7. To earn it, players must complete all seasonal Mythic+ dungeons at keystone level 10 or higher, within the time limit, and most importantly, in all three roles: tank, healer, and damage dealer.
As you can see below, you must earn all three achievements first before you get 
The Enterprising Dungeon Master.
