Sec. 70 read with rule 7 says A person liable to pay service tax is required to file ST-3.if exemption limit (turnover )has not crossed no liabilty to pay service tax will arise, hence no return is required.
Nil return is filed when assessee has turnover in excess of exemption limit i.e liable to pay service tax but in a particular half year no services are provided or no receipts are made ,in that situation nil return will be filed.
In case of sankalp there is no liabilty to pay service tax ( assumed exemption limit of Rs.10,00,000/- is not crossed) hence no need to file return.