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