You sure the cable is 100% on com3, as you can enter device manager and change com numbers, as when I plugged cable into windows 7 it set it as com5 and there is no com5 option, so I changed the com5 to a com3 in device manager and told vag-com to use com3.
Also the test will fail to find interface unless the vagcom cable is plugged into car with ignition in "on" position.