Switches have 2 positions open or close..
NC means normally closed
NO means normally open
The 3 pin allows the switch to be versatile in usage.
For example, it's almost always used in the NO position, if you take apart any logitech razer etc that use the 3 pin omron, the 3rd pin is soldered, but it's not connected to anything.
The 3rd pin in logitechs usually function as an anchor.
the REASON they use these ubiquitous 3pin switches is because they have a more distinct and powerful tactility and sound than the smaller 2 pin switches..