UFC's Valentina Shevchenko Says 3rd Fight W/ Rival Amanda Nunes Is Definitely Happening

Amanda Nunes is the ONLY woman to beat Valentina Shevchenko in the last 10 years ... and now the UFC flyweight champ tells TMZ Sports a 3rd fight with the Lioness is 100% goin' down!! We talked to the 32-year-old Shevchenko -- who's defending her…