one-third vs tierce

one-third and tierce both are nouns.

Word	Noun	Adjective	Verb	Adverb
one-third	Yes	No	No	No
tierce	Yes	No	No	No

As nouns, one-third and tierce are synonyms defined as:

Other synonyms of one-third include third.

one-third (noun)	tierce (noun)
one of three equal parts of a divisible whole	one of three equal parts of a divisible whole
	the cardinal number that is the sum of one and one and one
	the third canonical hour; about 9 a.m.