For words that have only three letters, there is absolutely no 4th letter. As an example with the word “add”, there isn't any additional letters Later on and thus must be the word "incorporate". The word “addict” is in the BIP39 wordlist but you might have utilized “addi” for “addict”. https://bookmarkfavors.com/story4345679/new-step-by-step-map-for-ledger-seed-phrase-bip39