if I just run Agent on PC-2(restart still not ok), I will see PC-2 usually changed status from on line and off line on PC-1 GMS windows. even when on connect between PC-2 and PC-3, it only get black screen and can't do any remote control.
Agent is for a different purpose than Host. You would want to use Agent for spontaneous, or even one-time support. You need someone sitting at the remote PC to run the Agent and tell you their ID and password. Once they close the Agent, the remote session stops.
Everything you can do with Agent you can also do with the Host, plus some features not available in Agent, such as starting as a system service.