I do not think they are the same as the goddesses. I think, as was said, that they are avatars or earthly representatives, but more like servants of the Goddesses, chosen soothsayers and people entrusted with some of their powers.
I tend to think of the Goddesses as being more mysterious - and more powerful. I mean, Veran posssessed the Oracle Nayru - somehow, I dought that would happen to a powerful creator goddess. Also, Ralph (a regular human) talks of being friends with Nayru since childhood - not something that could have happened between him and one of the three entities that created the world.
In addition to all that, Farore got ripped. I don't like to think of the oracle as being the all-powerful Goddess of Courage and Link's patron when she's... got a desk job? As the Oracle of Secrets? Yes, she got ripped off by Nintendo/Capcom there.