Here is the full list of all 20 Three Twenty One achievements.
Read through the viking book
Examining the football picture in the bedroom
Taking the bear in the text adventure
Finishing Castle without dying
Fail CodeCrashers
Collect all the books
Find the secret space room
Complete the game
Use the printer in the text adventure
Play Void
Taking the letter
Take the horse with you on the boat
Read through the friend book
Visit the PT area
Listen to the radio
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Listen to all audio recordings
Leave everything behind
Play 3:21 again