Interest rate on GST of March 2020

NIL Rate of interest if March 2020 return is filed within 15 days after 20 April.
9% interest if interest if return is filed after 15 days but before 30 June 2020. In this case, interest will be calculated from 16th day. Interest for first 15 days will be NIL.

No interest is applicable in your case for March month GSTR 3B .

In term of Notification 31/2020 CT , the Interest is applicable on taxpayer whose aggregate turnover in PRECEDING FY is more than 5 Crore.

Hence as per your query your aggregate turnover for Fy 2018-19 is nil ,( below 5 crore) interest is not leviable in your case.