Also called GSTR 3B, summary return, monthly GST return
GSTR-3B is a summary return. It does not list invoices. Instead it shows totals: tax on your sales, tax payable under reverse charge, input tax credit you are claiming, and the net tax you pay in cash. The payment is made along with it.
Monthly filers file it by the 20th of the following month. Quarterly filers under the QRMP scheme have a later date that depends on their state, and pay tax monthly through a challan. Filing late means a late fee and interest on any tax paid late.
A shop's March sales carry ₹54,000 of GST. Its eligible input tax credit, as shown in GSTR-2B, is ₹38,500. In GSTR-3B it reports both and pays the difference of ₹15,500. The numbers should agree with GSTR-1, or the difference will be questioned.