I would say that only A is correct, in your example.
1. It is three meters long ] fine.
B and C may be used as compound adjectives, e.g.
2. It is a three-meter-long fishing rod ] fine.
(Some people would dislike the partly hyphenated C.)
As for "two thirds/two-thirds", you'll find both forms. The prevailing tendency in everyday written English is to omit hyphens wherever there isn't a possibility of confusion. But in a more serious publication, you would probably find it written "two-thirds".