For words that have only 3 letters, there isn't any 4th letter. Such as with the word “increase”, there are no a lot more letters afterwards and thus has to be the word "add". The word “addict” is during the BIP39 wordlist however, you would have utilized “addi” for “addict”. https://captainbookmark.com/story18860790/the-invalid-bip39-phrase-diaries