The Shadowlands Season 3 Mythic+ affix has been found in the files of the Eternity's End PTR and is called "Encrypted".
While the first 2 seasons and Legion Timewalking all feature similar +10 affixes, this is slightly different as mobs will occasionally spawn during combat, with seemingly no control over the spawn timings.
Season 3 Mythic+ dungeons are not currently available on the patch 9.2 PTR, although should become available over the next few weeks for players to begin trying before the official release sometime in the new year.
First Look At The Shadowlands Season 3 Mythic+ Affix
There is also no information on what the powers will be if it works like the current anima system or something less permanent like the Legion Timewalking's Infernal Affix.
Players will also need to wait and see if the summoned Automa counts as mob % in each dungeon, as they will make certain groups take longer to clear, and we do not even know how often they will spawn in each dungeon.
This will help keep the dungeons fresh, as well as the addition of Tazavesh, the Veiled Market making its way to the mythic+ system, which will be split into two dungeons.
We will know more over the next month as testing on the Eternity's End PTR continues to evolve every week, it's just a question if we see these changes before the holidays or not.