Can you show why? Not a certificate - that is essentially meaningless. Or less. Can you show other period examples of signatures that match that Linda for example? If authentic, with several signatures, some will resemble the known variants. If authentic.
I don't believe the album posted by Erik is authentic. Sorry. Pretty typical "tourist shop" forgery, IMO.
I think I've seen the exact same album "signed" by those three individuals in the same matching pen with the same style signatures for sale in a certain autograph store in Las Vegas.