Три сотні гривень it is also must be good
That would be acceptable, but would sound strange.
It sounds fine, because it is synonymous
You for sure can say it this way.
For me as for the native speaker it would sound a little bit stranger than "триста гривень" though.
I think that "hryvnyas" is not the most easy and not the only way to write the valute. It may also be "hryvnias" or "hryven" or even "griven"