For words that have only 3 letters, there isn't a 4th letter. As an example with the word “add”, there are no additional letters afterwards and so has to be the word "incorporate". The word “addict” is in the BIP39 wordlist but you would have applied “addi” for “addict”. This https://e-bookmarks.com/story4770038/fascination-about-invalid-bip39-phrase