Let's understand definition of supply given in CGST Act.
all forms of supply of goods or services or both such as sale, transfer, barter, exchange, licence, rental, lease or disposal made or agreed to be made for a consideration by a person in the course or furtherance of business;
So its a supply if consideration is involved. In you case supply does not involve consideration because you are not receiving any money against samples sent.
So, you need not to show it in GSTR-3B.
Again, since it is not a supply it will not be considered any where in GSTR-1. However for accurate reporting you can include details in HSN summary table.