GSTR 3B is a simple return form introduced by the CBEC for the month of July and August. You must file a separate GSTR 3B for each GSTIN you have. Do note that you do not have to provide invoice level information in this form. Only total values for each field have to be provided, this value must be for the month for which you are filing.

The due date for filing GSTR – 3B is 20th day of the following month
For ex. GSTR 3B for August 2017 is due on 20th September and GSTR 3B for September 2017 is due on 20th October
GSTR 3B is not applicable starting the month of September, regular returns in GSTR-1, GSTR-2 and GSTR-3 have to be filed from September.

Every person who has registered for GST must file the return GSTR 3B.
This return form must be filed and submitted by you even if you do not have any transactions. However, the following registrants do not have to file GSTR 3B.
Input Service Distributors
Composition Dealers
Suppliers of online information and database access or retrieval services (OIDAR), who have to pay tax themselves (as per Section 14 of the IGST Act) Non-resident taxable person.