Query about treatment of below case in Gst return 3b.

it will depend whether you have applied for voluntary registration or compulsory registration.....
if you have applied voluntarily and your turnover upto 17 july (as rc issued on 18) is below 20 lakh, than you need not to give detail in 3rb of sales before 18 july, as your liability start from 18 july.....
on the other hand if you have registered compulsorily than you have to provide detail of sale from the date when you became liable to be registered....

wht is voluntry and complsry

If a person wants to get himself registered though he is not required to get himself registered It is called Voluntary Registration

If a person crosses the limit of supply of 20 lakh, he is bou nd to get himself registered. It is called compulsory registration.