I've had a brief look at the instructions and it tells you exactly what to type so provided you're typing it exactly as shown (but not typing [enter]), then it should work if you've followed the previous instructions.
The crucial thing is that you've copied the SP3 installer file into C:\sp3. If you've put it anywhere else then the command prompt window will be looking in the wrong place for it. (It may also be case sensitive, I'm not sure, so if you've named the folder SP3 as opposed to sp3 that might cause a problem. Just make sure the path you type into command prompt is the same as where the file is).