Your previous years NEET score is valid for 3 years only if you want to study MBBS abroad because now NEET is mandatory for studying MBBS abroad. But if you want to study MBBS or BDS in India, your score will be valid only for the year that you wrote NEET for. For example, your NEET 2019 score will be only useful and valid for 2019. It cannot be taken forward to 2020 and it is not used to calculate or consider the best of 2 or 3 NEET scores.