For words that have only 3 letters, there is not any 4th letter. For example with the word “increase”, there isn't any a lot more letters afterwards and so should be the word "add". The word “addict” is while in the BIP39 wordlist but you would've applied “addi” for “addict”. https://thesocialdelight.com/story4658489/5-essential-elements-for-bip39-explained