threepenny vs two-a-penny

threepenny and two-a-penny both are adjectives.

Word	Noun	Adjective	Verb	Adverb
threepenny	No	Yes	No	No
two-a-penny	No	Yes	No	No

As adjectives, threepenny and two-a-penny are synonyms defined as:

Other synonyms of threepenny include sixpenny, tuppeny, twopenny-halfpenny, twopenny.

threepenny (adjective)	two-a-penny (adjective)
of trifling worth	of trifling worth
used of nail size; 1 1/4 in long