Missed sales invoice

Show such invoice in the GSTR-3B of Aug 2018 with interest

You can rectify/add any invoice which was ommitted in the relevant period in any succeeding month but before the September of next year

As in your example: If any bill was ommitted for the tax period of 2017-18 you can add/ rectify upto the filling of return for the period of September 2018

As regard to GSTR 1 you can add it in
1. B2B invoices if not shown earlier
2. Amend B2C for the month of August 17 in the return of August 2018 if it is Monthly or in the Return of September 2018 if it is quarterly
3. Amend B2B invoice if it is already shown and you have to rectify it.

ok so if I now show the invoice in the return of August, the GSTR 3B of August should be inclusive of the taxable amount of such bill. Right?

yes right.. n also pay interest on that while paying tax for aug month

Yes, as far as I know.

And how do I calculate the interest amount?

amount of tax * 18% * no. of days for which thr is delay / 365

if we have already  Input Tax Credit of GST , then also we needed to pay Interest on Addition Invoices added in Next month 

.if yes. how to calculate no of days  on for delay intt .