Gst 3b and gstr 1 query

If he added sales invoice in GSTR 1 but not reported in GSTR 3B than you can report that amount in current GSTR3B in output tax Column along with current month's sale and Pay tax accordingly...
If you forgot to take ITC or ITC not taken relating to invoice of F.Y 2019-20 than you can take that ITC in current month GSTR 3B..

You have / can do all this on or before 20th October, 2020 (i.e due date of filing GSTR 3B of September, 2020)

Ok sir thank u so much

Welcome...........

A useful article on September 2020 compliances under GST - https://hiregange.com/a/fy-201920-gst-compliance-action-points