Great Britain vs the three estates

Great Britain and the three estates both are nouns.

Word	Noun	Adjective	Verb	Adverb
Great Britain	Yes	No	No	No
the three estates	Yes	No	No	No

Great Britain (noun)	the three estates (noun)
an island comprising England and Scotland and Wales	a major social class or order of persons regarded collectively as part of the body politic of the country (especially in the United Kingdom) and formerly possessing distinct political rights
a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland; `Great Britain' is often used loosely to refer to the United Kingdom