How do you get the Season 2016 - 3v3 Challenger 2?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Challenger in the 2016 Ranked Season in 3v3.”
Is the Season 2016 - 3v3 Challenger 2 still available?
No. The Season 2016 - 3v3 Challenger 2 was a ranked reward: to get it you had to reach Challenger in Twisted Treeline 3v3 during Season 6 (2016). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
Season 2016 - 3v3 Challenger 2 is one of the rarest LoL summoner icons.
