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