In the BUSY,you can manually enter invoices only, you need to go to the transaction menu and add sales, purchases, and other transactions.These invoices will be automatically populated in your GST Reports and returns,as GSTR3B Also.If still not showing figure in GSTR3B Report,may be it is possible that the figure for vouchers has not been updated, so you can Go to House keeping menu there is a option is Update voucher GST summary, After taking a backup, please run it, and your GST reports will be updated. Then check your GSTR3B Report the figures will be updtaed.