It's called a symlink (symbolic link). But if the entire Windows user Documents library has been shifted to D by way of the operating system, then the game folder is already in the proper place. The game doesn't care where Documents is located, it just needs to know to look in Documents, wherever Windows tells it that it is, to find the TS3 user game folder.
The reason that very small C drives (SSD) on the order of 128 GB are combined with much larger D drives (HDD) as a common system configuration is that SSDs are much more expensive than HDDs. So the idea is to provide the home user with an SSD for Windows and a slower regular drive for just about everything else to lower the cost of the entire system.