Hint Memo 3
From Starfy Wiki
Hint Memo 3 (Japanese: ヒントメモ3) is an item in Densetsu no Starfy 2. It is found in Pitch Dark Cave (Stage 6-5). It is one of three notes that contain a password. When combined together, they will open the red, green and blue doors set up by Puchi Ogura #6 that block Starfy's path. The other two notes are the Hint Memo 1 (Japanese: ヒントメモ1) and the Hint Memo 2 (Japanese: ヒントメモ2).
The Hint Memo 3 is found in through the door near the north-east corner of the room containing the red, green and blue doors.