For words that have only 3 letters, there isn't a 4th letter. One example is with the word “increase”, there are no additional letters afterwards and therefore have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would've made use of “addi” for “addict”. https://letsbookmarkit.com/story18830311/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting