For words that only have 3 letters, there is absolutely no 4th letter. Such as with the word “add”, there won't be any more letters afterwards and so should be the word "add". The word “addict” is during the BIP39 wordlist but you would've made use of “addi” for “addict”. https://fatallisto.com/story8574411/ledger-seed-phrase-bip39-no-further-a-mystery