I am a PGD Diploma 2021 batch, I've 3 backlogs I wasn't able to attend supply exams. When they will be held this year?

The supplymentary exams mostly depends upon the university. The university fixes the dates accordingly for the exams. So try contacting your college as they will get the information and based on that you will be informed by them beforehand for the payment of supply fees and for exams also.