gstr 1

If turnover is below 1.5Cr you should have select option for quarterly. Once option selected monthly you can not reverse the option till march 2018. Now you have to file GSTR 1 monthly.

GSTR 3B are monthly irrespective of turnover 

You can voluntarily choose to file manually if your Turnpver is less than INR 1.5 Crors in previous year or likely to be in the current year.

There should not be any issue with it.