Can you unsubscibe one computer to register a new one

Hi  i have a question ,i have a subscription that covers three computers and  until recently  we only had one desktop computer which was registered, the kids are getting older so recently  i bought three notebooks ,i have registered two  but now need to register the third ,the desktop is no longer connected to the internet so needs no protection,can i un register this pc to allow the third notebook to be registered ,i hope someone can shed some light on this for me. thanks Ian