"avrdude: ser_open(): can't open device "\.\COM3": The system cannot find the file specified.
Problem uploading to board. See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.
This report would have more information with
"Show verbose output during compilation"
enabled in File > Preferences."
what was that error plzz help me
Check that the Uno is actually present on port COM3. Use Device Manager to check this.
Also verify that the correct board is selected. I had the same problem once when trying to upload a sketch to my Uno with the wrong board selected.
Hope this helps. 
This error can also happen if you have a board piggy backed onto your arduino. I simply unplug the piggyback board then compile the firmware. After a successful compilation remount the board.
Andy