GST 3B Returns fore sub brokers

Sub broker is person acts on behalf of broker as agent , So as per section 2(5) CGST act you fall under defination of Agent .
Securities are not cover under the defination of goods in GST so its out of purview of GST but services provided in relatiing to same is taxable in GST (Sec.2(102))

So you are liable to deposit GST on the payment reced in form of brokerage from company