How do you get the Season 2016 - 3v3 Master?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Master in the 2016 Ranked Season in 3v3.”
Is the Season 2016 - 3v3 Master still available?
No. The Season 2016 - 3v3 Master was a ranked reward: to get it you had to reach Master 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 Master is one of the rarest LoL summoner icons.
