GSTR1

you can file your GSTR 1 for April 18 now

missing 3 bills of April 18 are purchase or sale Bill's?
its sale bill you can file the 3 Bill's in December or January GSTR 1

Agree with arun sir answer

GSTR1 is related to outwards supplies.
The query asked may be of sales invoice.
If you are not filed GSTR1 of April 2018, then show all the invoices (including missed out 3 invoices) in April 2018 month GSTR1.
In 3B, you have shown missed invoices in nov 2018 then you are liable to pay interest @ 18%p. a.