For words that only have 3 letters, there isn't any 4th letter. One example is with the word “insert”, there isn't any much more letters Later on and thus should be the word "add". The word “addict” is in the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://bookmarknap.com/story9861219/bip39-seed-phrase-mnemonics-generator-an-overview