For words that have only 3 letters, there isn't any 4th letter. By way of example with the word “include”, there isn't any additional letters Later on and thus have to be the word "add". The word “addict” is during the BIP39 wordlist but you would've employed “addi” for “addict”. https://www.anobii.com/en/01d109953642f9cd99/profile/activity