I need help getting my computer to talk to my Arduino Uno
There's an easy way to force windows to drop anything on that port and that's to rename/reassign it. I've done it myself when the serial gets abruptly disconnected and windows doesn't handle it. To do this you need to:

1) Go to Control Panel-> Device Manager 

2) Look for the COM port and right click, go to properties. 

3) In the Advanced tab there is a button that is also called advanced. In it you can reassign the COM3 to another COM. Disregard any warning that "This port is being used" because since you don't have anything connected to those ports they're not being used.

4) Once you reassign it (and press OK to whatever it complained about to force it), then connect and disconnect the Arduino. It should show up in the new port and work.