For words that have only 3 letters, there's no 4th letter. As an example with the word “incorporate”, there won't be any additional letters Later on and therefore need to be the word "incorporate". The word “addict” is from the BIP39 wordlist however , you would've utilized “addi” for “addict”. https://bookmarkangaroo.com/story18933512/a-review-of-bip39