two-a-penny vs threepenny

two-a-penny and threepenny both are adjectives.

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

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

Other synonyms of two-a-penny include sixpenny, tuppeny, twopenny-halfpenny, twopenny.

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