For words that only have 3 letters, there isn't a 4th letter. Such as with the word “add”, there won't be any additional letters Later on and so have to be the word "increase". The word “addict” is within the BIP39 wordlist however you would've utilised “addi” for “addict”. In https://tealbookmarks.com/story18792566/the-definitive-guide-to-bip39-seed-phrase