Is it permissible for this person to agree with the company on receiving a commission without my knowledge? And must he return the commission he received from me, or the one he received from the company which I paid as part of the purchase cost? And am I entitled to demand of him not to receive a commission from the company exceeding 3%?

If a person acts as your agent for a purchase, and you pay him a fee for his agency, then what the seller gives him does not belong to him; rather, he must hand it over to you. This is because an agent is entrusted (with a trust), and whatever he receives due to the agency becomes part of the price and belongs to the owner of the money (the principal), and he must return it to him.