How unique is canvas fingerprint?

Question

Canvas

Why do 3 different hardware and software machines have the same canvas fingerprint?

Answer 1

Because no known method for obtaining a device fingerprint guarantees the uniqueness of the received key, incl. and using canvas. Browsers simply don't take on this responsibility, and the reasons may vary depending on the implementation of each particular version of the application.
Moreover, browser developers intentionally generalize information about the machine available to JS in order to ensure the privacy and security of users. So 100% rely on such things is not worth it anyway.