GSTR 3B

1. If we have to file GSTR 3B for July and August then do we need to file GSTR 1.2,3 also for July and August?2. In case of Software Exports all export invoices need to be filled in GSTR 1 in EXP. what should be the shipping bill no?3. If Input Tax Credit is higher than Output in CGST and SGST but lower in IGST can this is set off?