third-rater vs doer

third-rater and doer both are nouns.

Word	Noun	Adjective	Verb	Adverb
third-rater	Yes	No	No	No
doer	Yes	No	No	No

As nouns, doer is a hypernym of third-rater; that is, doer is a word with a broader meaning than third-rater:

Other hypernyms of third-rater include actor, worker.

third-rater (noun)	doer (noun)
one who is third-rate or distinctly inferior	a person who acts and gets things done