Well spotted baggss! Yes, the OP stated that it's only USB Port 3 that isn't working [I missed that - smack!]. To Suku, try swapping the other devices, eg. keyboard and mouse about, using either of them in Port 3.

As far as the terminal command 'diskutil unmount' is concerned, and dealing with the simpler things first, try reformatting the flash drive using your other Mac. You could preserve the contents of the drive to the other Mac as well.

If USB Port 3 continues not to respond to anything inserted, then we're looking at hardware repair. It's possible that could be achieved with Disk Utility from the original Install disk 1.