The secret Discovery Battlestar of the third week of Season 8 is located next to the temple east of the volcano.
To actually make the battlestar appear, you must first complete a full set of weekly challenges (weeks 1-10).
Once you’ve completed a set of them, this secret battlestar can be picked up and the challenge “Find the Secret Battlestar in Loading Screen 3” is displayed in the Discovery Challenges. It will reward you with a full battle pass level.