What platform is the game 3Xtreme available on?
what platform refers to platform_name; 3Xtreme refers to game_name = '3Xtreme';
SELECT T2.platform_name FROM game_platform AS T1 INNER JOIN platform AS T2 ON T1.platform_id = T2.id INNER JOIN game_publisher AS T3 ON T1.game_publisher_id = T3.id INNER JOIN game AS T4 ON T3.game_id = T4.id WHERE T4.game_name = '3Xtreme'