For words that only have 3 letters, there isn't any 4th letter. For instance with the word “increase”, there isn't any a lot more letters afterwards and so needs to be the word "insert". The word “addict” is within the BIP39 wordlist however , you would've used “addi” for “addict”. https://gorillasocialwork.com/story21889096/bip39-seed-phrase-fundamentals-explained