What GSTR-3B is
GSTR-3B is the summary return in which you declare your outward supplies, the input tax credit you claim and the tax you pay for a period. Every regular GST taxpayer files it, monthly or quarterly under the QRMP scheme, including a nil return for a period with no activity.
Monthly filers file by the 20th of the following month. QRMP filers file each quarter, by the 22nd or 24th of the month after the quarter depending on their state. The government can extend a due date by notification, so check the current date before you file.