These three platforms share the same processor, but there are a few differences with regard to hardware peripherals. Both Beaglebone Green versions (BBG and BBGW) support the open-source click-in "Grove" hardware modules from Seeed Studio. BBG versions are better suited to embedded applications than Beaglebone Black, but the BBG or BBGW can still be used as a standalone Linux computer, although without the HDMI support for monitors that is on Beaglebone Black. Nevertheless, both of the Green versions support all of the hardware add-on modules (called "capes") as the Beaglebone Black, including capes that can support a monitor.
BBG and BBGW boards come with a small instruction manual and a 6 inch USB standard A to USB micro B cable. Everything else, as well as the system reference manual, is stored on the board itself, which acts like a USB thumb drive when plugged into a Windows or Linux computer, so installation and boot up are easy.
A detailed comparison of all three boards follows in the link below. There is a much larger scale comparison of several open source hardware boards in a large expandable table that is intended to facilitate OSHW board selection (based on project requirements) at the Open Source Hardware Applications site.
View Comparison Chart