Correcting gstr-3b mistakes of fy 17-18 after sep-2018 gstr-3b is filed

Dear Experts,

After completion of Tax Audit and Finalization of Accounts, certain mistakes were noted in calculating sales and purchases, (which affects GST liability & ITC reported in GSTR-3B for previous FY 2017-18). Still computing nett differences to check if there is any additional tax liability or unclaimed ITC.

So,

1) Now that, the September 2018 GSTR-3B is filed, can these figures still be adjusted in subsequent GSTR-3B ?

2) If Yes, then till which month's GSTR-3B  can they be adjusted ?

3) If NO, then how / where to adjust the same ?

Kindly help clarify this issue.

Thanks.