How do you get the 2016 Worlds Tier 3 Chiefs eSports Club?
It was a mission or event reward. Riot's description in the client reads: “This icon was the highest tier reward players could earn for participating in the Worlds 2016 in-game festivities.”
Is the 2016 Worlds Tier 3 Chiefs eSports Club still available?
No. The 2016 Worlds Tier 3 Chiefs eSports Club was the top reward tier of the Worlds 2016 in-game event: you had to progress that far through the event to earn it. Riot does not run past events again, so it cannot be earned anymore.
