Basic info:
To active stage 3, you need purchase Elder Pact from upgrades. When stage 3 is, In building grandma will change image to angered grandma image. And background will change to Angered Grandmas.
Advanced Info:
Unlocks 2 upgrades: Elder Covenant , Elder Pledge
When elder pledge acquired grandmapocalypse ends for 30 minutes
Elder pledge has infinite uses.
When Elder Covenant acquired grandmapolypse end for random time minimum: 1 Hour
And elder covenant is toggleable and has infinite uses.
If you need know more about stage 3 post in comments.