GSTR-3B is the monthly GST return to be filed for the months July to December only.
GSTR-1 is the return/statement which need to be filed for all months, here you need to provide invoice details of all outward supplies during the month.
GSTR-2 is the return/statement for inward supplies. This will be auto populated based on GSTR-1 filed by your suppliers.
GSTR-3 is the main return for each month. It will be auto-populated from GSTR-1 and GSTR-2. You need to setoff tax liabilities and submit this return.