For words that have only three letters, there isn't a 4th letter. As an example with the word “increase”, there are no much more letters afterwards and therefore need to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however you would've employed “addi” for “addict”. https://scrapbookmarket.com/story18873324/fascination-about-bip39-english-words