Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
third [θəːd] adj тре́тий#
n (fraction) треть f, одна́ тре́тья f adj (AUT) (also: third gear) тре́тья ско́рость f (BRIT) (SCOL) (degree) дипло́м тре́тьей и́ли ни́зшей сте́пени
a third of треть +gen, тре́тья часть +gen see also fifth
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.