How do you get the Season 3 - Platinum?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Platinum in the 2013 ranked season in any queue.”
Is the Season 3 - Platinum still available?
No. The Season 3 - Platinum was a ranked reward: to get it you had to reach Platinum during Season 3 (2013). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
