What are the three rules the "Receiver of Memories" had to follow?
Technically, there are more than three rules that the Receiver of Memories must follow. In Ch. 9 Jonas receives one sheet of instructions that gives him eight distinct rules. However, there are three rules that Jonas seems to focus on the most because he can really foresee how they will affect him.
Rule #3 "startled him" because it exempted him from "rules governing kindness." He no longer had to worry about what he was going to say to people, which he had grown accustomed to his entire life.
Rule #6 "unnerved him" because he had only experienced brief pain and when he had, he was immediately given pain medication and didn't feel it any longer. But for that short time, the pain was excruciating. This made him think about if or how he may need it again and if his pills for the "stirrings" qualified as being prohibited; he realized they did not.
Rule #8 simply said, "You may lie." This was literally inconceivable to Jonas because he had never intentionally lied nor did we get the impression it had ever crossed his mind. He then wondered if other twelves might have also received this same "rule" in their instructions and if so, how many members of the community might not be telling the truth.