There is no official "World's Hardest Game 3." The series consists of two main Flash-based platformers created by the developer "The Impossible":
Official versions: Both games are playable on major Flash game portals like Kongregate, Newgrounds, and Armor Games. Use their search functions to locate them.
Unofficial "World's Hardest Game 3" titles: Searching Google or YouTube for "World's Hardest Game 3" will yield fan-made or unofficial clones. You may also find user-created versions on platforms like Roblox.
The original two games offer a genuine sense of accomplishment upon completion. Exercise caution when playing games from lesser-known websites, as they may contain malicious content.