For words that only have 3 letters, there is absolutely no 4th letter. Such as with the word “add”, there aren't any extra letters Later on and therefore need to be the word "add". The word “addict” is from the BIP39 wordlist however , you would've utilized “addi” for “addict”. https://bookmarkingquest.com/story18787049/little-known-facts-about-bip39-seed-phrase-list