How do you get the Season 2017 - Flex 3v3 Diamond?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Diamond in the 2017 ranked season in Flex 3v3.”
Is the Season 2017 - Flex 3v3 Diamond still available?
No. The Season 2017 - Flex 3v3 Diamond was a ranked reward: to get it you had to reach Diamond in Twisted Treeline 3v3 during Season 7 (2017). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
