Are you certain that COM3 is the port of your USB-serial adapter? You can test that by unplugging the usb cable, wait a little while, then check if the COM port has disappeared from the Tools > Port menu.
Are you certain that COM3 is the port of your USB-serial adapter? You can test that by unplugging the usb cable, wait a little while, then check if the COM port has disappeared from the Tools > Port menu.