Will getting a multiple entry permit extend my 3-month non-B visa when I apply for a one-year extension later?

A multiple re-entry permit is only valid until the expiration of your current visa entry stamp. If you obtain a multiple re-entry permit on your 3-month non-B visa, the permit will only cover the period of your initial 90-day entry. Once you apply for a one-year extension, the multiple re-entry permit will not extend. If you need to leave and re-enter Thailand before the 90 days is over, consider obtaining a single re-entry permit to maintain your visa status.