For words that only have 3 letters, there isn't a 4th letter. As an example with the word “incorporate”, there isn't any additional letters afterwards and thus must be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would've applied “addi” for “addict”. Artwork is https://tornadosocial.com/story4387640/5-simple-techniques-for-bip39-word-list-pdf