Gstr 1 query

If You not entered / missed some invoice in GSTR 1 or GSTR 3B than you can add that Invoice in current month GSTR 1 or GSTR 3b as the case may be.

If you already entered in 3B but only not entered in GSTR 1 , than enter that invoice in GSTR 1 only. No need to pay GST as you already paid via 3B.

But not entered in 3B also than you have to disclose that in current 3B and pay tax according.

Follow Sagar Ji advice 

For GSTR Compliance