Hi! For several reasons, there is no native "guestbook" plugin in X3. A guestbook requires strong security, strong spam protection, possibly user-registration, and definitely a moderation tool admin that allows you to manage/delete guestbook posts. It's not really feasible to build a "simple" guestbook function.
Instead, considering "guestbook" is really just "comments", personally I would use
Disqus for this (which is supported natively in X3). Disqus is a free "comments" service, which already has strong protection and dashboard to control settings and moderate incoming comments. There is an example of Disqus comments on this page (scroll down):
https://demo.photo.gallery/examples/features/post/
Basically, you would create a page "guestbook", add Disqus comments plugin, and have it work pretty much like a guestbook.