This is difficult to explain only by words. See the picture below. The goal is to make PC3 part of the network where PC1 and PC2 belong. Of course, assume that PC2 is on when PC3 is on. The operating systems are Windows.
The reason why I do not connect PC3 to the router is that there is a wall, so it will be difficult to connect PC3 to the router directly. PC2 is directly connected to the router, but it was difficult to do so, so I do not want to do it again for PC3.