For words that have only three letters, there is not any 4th letter. By way of example with the word “insert”, there isn't any additional letters afterwards and so have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would've made use of “addi” https://socialbuzzmaster.com/story4615059/examine-this-report-on-bip39-mnemonic-phrase-tool