GST3 Return Means

There is no form as GSTR3 as of date. In place of that we have GSTR-3B which is a summary return and it needs to be filled. Some figures are autopopulated but they can be changed.

No such GSTR 3 Return is operational as of today.
If talking about GSTR 3B, then it is a summary return mainly for payment which is mostly autopopulated these days.
Sales is being auto populated from GSTR 1 and purchases from GSTR 2A/2B.
You can do corrections in the auto populated figures.
Different due dates are there for different tyoes of taxpayers depending upon the turnover/state of registration/QRMP.

Currently the due date is 20th of the following month for taxpayers having turnover of more than 5 crores annually.

Message for more clarification.

GSTR-3B is a monthly return filed for paying the liability in cash or via credit. Yes it is auto-populated from sales shown in GSTR-1 and ITC available as per GSTR-2B. Yes you can do correction. But if your liability is reducing or ITC is increasing due to corrections then GSTN may ask you the reason for the same. Due date for GSTR-3B is 20th of subsequent month.