For words that have only 3 letters, there is no 4th letter. For example with the word “add”, there won't be any additional letters afterwards and therefore need to be the word "include". The word “addict” is while in the BIP39 wordlist but you would've employed “addi” for “addict”. BIP39 https://socialmediainuk.com/story20408164/the-5-second-trick-for-ledger-seed-phrase-bip39