gstr1 and gstr3b

You can provide consolidated details for the 3 months in your GSTR 1

For this categorise your all sales into B2c and B2B and in HSN wise too (For the three months)

GSTR 3B is monthly return. You can as per Your monthly transactions. If sale transactions in a month then fill details, If not then them don't be fill or file nil return in case no more details...

&

You are under Quarterly mode for filling GSTR 1 return then file as per Your 3 months total transactions as per Your invoice...

No more problem In case no sale transactions in any of the month. But, You can keep Your invoice series continuously...

GSTR 3B is to be filed monthly in all cases. However, GSTR 1 can be filed quarterly in few cases.

Suppose you are filing GSTR 1 for the quarter Oct to Dec 2018 then select December 2018 in GSTR 1 return filing option.

The GSTR 1 will contain all the sales and purchase made between Oct to Dec 2018.  

GSTR-1 return only for Sales and Sales Return...
Not for Purchase Details and it's related activities...