Question about subpanel install in a shed
I just looked and VA is on the 2011 NEC , I thought the change to 4 wire was back in the 99 code. So I would check with your local inspector to find out what the local requirements are (I would want 4 wire and ground rod). Very strange that a 2014 sub would be set up this way unless allowed by VA code modifications. 

Added after op coment: you will need to update the garage to the 4 wire with ground rod to meet today's code if you want to pull a sub from the garage. I have used pipe and water hose to tunnel under driveways and sidewalks many times in the past so you don't have to cut them depending on your soil conditions its a bit messy but cheaper than cutting in most cases.