GST vide Notification No. 08/2021 dated 1st May, 2021 has extended the due date for filing Form GSTR-3B for the months of March, 2021 and April, 2021. Please refer the link below for the extended dates:
https://cbic-gst.gov.in/pdf/central-tax/notfctn-08-central-tax-english-2021.pdf