Few sale invoices missed while filing GSTR 3B for April 2019

If it is B2C bill and you have an intention or bound to file GSTR 9, then you can show the same in GSTR 9 for that year and pay the same via DRC-03
Alternatively, you can show it in the most possible month of now in GSTR 3B and if you had filed GSTR1 nil too then you can ammend B2Cs in the currently available month (try to do this in this month as the last available month is september)..
If it is B2B bill then in GSTR 1 you can enter the bills with old date. So, upload the same in GST website
While paying this amount pay interest @ 18% p.a. on the tax amount to avoid any future complications..

You can add that sale bill in outward Supply of GSTR 3B of current month and Pay tax along with interest (Required to do this in September GSTR3B or earlier month i.e before 20th October)

And in Current GSTR 1 also you can add that sale bill as normal' but with old date..
You are required to do all this in September, 2020 return or in earlier return like August return or July Return if it's pending.