"I'm rather tired" = "I'm fairly tired."
These are roughly equivalent, though I would consider "fairly tired" more tired than "rather tired."
"Quite" presents its own set of problems.
"I'm quite tired." will mean roughly the same thing as the above.
BUT, "quite" is also used to mean "entirely." For example:
You are quite wrong/right about that! (entirely wrong or right)
You will find this usage with adjectives that don't present a gradation: think of a whole continuum of being tired, but only two alternatives in being right or wrong.