For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “add”, there are no extra letters Later on and thus should be the word "insert". The word “addict” is inside the BIP39 wordlist but you might have made use of “addi” https://bookmarkingace.com/story18824653/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase