Who came up with the trident symbol found on all USB devices and what
do the triangle, the square and the circle at the end of the three
prongs symbolize?

Unfortunately, I could find no reference to what the artist(s) had in
mind when they created the USB trident logo.

The USB specification, including the logos, is maintained by the USB
Implementers Forum (USB-IF).
http://www.usb.org/

This PDF file includes the logo and specifications for USB.
http://www.usb.org/developers/docs/icon_design.pdf

Another resource I checked (but, came up empty) was this:
http://www.usbman.com/

I include the references here in case the forums on those sites can
offer you more help.

The USB square circle triangle Trident symbol is most likely symbolic
of the different devices that can be chained to a USB connection. 
Prior to USB, only SCSI had this capability.  The square, circle, and
triangle don't appear to have any particular significance, other than
signifying different connected devices.  The Trident symbol itself
signifies the different data paths available to connected devices for
connection to the primary computer.