How do you get the Season 2015 - 3v3 Gold?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Gold in the 2015 ranked season in 3v3 Team Ranked.”
Is the Season 2015 - 3v3 Gold still available?
No. The Season 2015 - 3v3 Gold was a ranked reward: to get it you had to reach Gold in Twisted Treeline 3v3 during Season 5 (2015). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
