For words that have only 3 letters, there isn't a 4th letter. By way of example with the word “include”, there aren't any additional letters Later on and thus has to be the word "incorporate". The word “addict” is within the BIP39 wordlist but you would've employed “addi” for “addict”. https://artybookmarks.com/story18683166/bip39-seed-phrase-fundamentals-explained