How do you get the Master's Challenge Season 3?
It was a ranked reward. Riot's description in the client reads: “This reward commemorates the top 2000 players on Master's Challenge shard.”
Is the Master's Challenge Season 3 still available?
No. The Master's Challenge Season 3 was a ranked reward: to get it you had to reach Master during Season 1 (2000). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
Master's Challenge Season 3 is one of the rarest LoL summoner icons.
