Hi Cristine,
You'll want to request to be on the Trusted List of your 3rd great grandfather's profile. On his profile on the right hand side you should see a link you can click to request that. That will send a message to the profile manager who can then add you. Be sure to mention your relationship to William ~ chances are good the profile manager is probably related to you somehow so maybe you will also make a cousin connection! :)
Eowyn