Do you mean that there are already 3 computers listed in the Dorico 4 section? I mean the section of MySteinberg that looks a bit like this:
If so it would be useful to us to know why our systems think you have been using 3 different computers when you have only been using one. Would you be able to zip up your Activation Manager directory and attach it here? It will be found in /Users/yourusername/Library/Application Support/Steinberg/Activation Manager on Mac, or %appdata%\Steinberg\Activation Manager on Windows.