What do the 3 symbols (circle, square, triangle) in the USB Type-A plug represent?
They don't seem to have much meaning from an electronics point of view and a few sites I checked such as Famous Logos seem to indicate it's more of an artistic impression:

Trident in the USB Logo


  The USB logo was said to be modeled to mimic the trident of Neptune, the mighty Dreizack. Although the trident before may symbolize power and authority, the USB trident is more commonly attributed to the technological ‘power’ one can get from attaching the USB to the computers and other devices.


Shapes in the USB Logo


  The shapes at the tip of the three-pronged spear (triangle, square and circle) in the USB logo are placed there to signify all the various peripherals that can be attached if the standard devices are used. It also denotes how interconnected the world has become, thanks to the technological innovations that the USB has imparted.