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