How do you get the Season 3 - Challenger?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Challenger in the 2013 ranked season in any queue.”
Is the Season 3 - Challenger still available?
No. The Season 3 - Challenger was a ranked reward: to get it you had to reach Challenger during Season 3 (2013). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
Season 3 - Challenger is one of the rarest LoL summoner icons.
