For words that only have 3 letters, there isn't a 4th letter. For example with the word “increase”, there aren't any more letters afterwards and thus must be the word "insert". The word “addict” is while in the BIP39 wordlist however, you might have made use of “addi” for “addict”. https://kbookmarking.com/story18935462/5-essential-elements-for-bip39-explained