GSTR3 is the actual return which will be containing data of inward supplies from GSTR2 of the registered person and details of outward supply from GSTR1 of the registered person.
The person has to self assess his tax liability and pay tax accordingly using Input tax credit or through bank.
The government has issued a notification notifying the last dates for filing of GSTR2 for the months of July and August. The last dates are:
For July, 2017: 11th to 15th September, 2017.
For August, 2017: 26th to 30th September, 2017.
Find the notification below: