For words that have only 3 letters, there's no 4th letter. For instance with the word “increase”, there won't be any more letters Later on and thus has to be the word "insert". The word “addict” is during the BIP39 wordlist however you would have applied “addi” for “addict”. A https://hylistings.com/story20086999/the-5-second-trick-for-ledger-seed-phrase-bip39