Hydra Core namespace URI / @context ?

In the Hydra spec I see hydra being represented as:

1. "@context": {
  "hydra": "http://www.w3.org/ns/hydra/core#",

2. "@context": "http://www.w3.org/ns/hydra/context.jsonld",

3. "@id": "http://www.w3.org/ns/hydra/core",

Which one is correct? How does a JSON-LD client knows that (presumably?)
all three URIs are the same thing?

If I write a pure Hydra document with a single @context, what should I use:

1.  "@context": "http://www.w3.org/ns/hydra/core#",

2. "@context": "http://www.w3.org/ns/hydra/context.jsonld",

What's the difference? And which one is "easiest" for a (rather naive)
client to consume, but still ability to distinguish properties from
different namespaces? (Without being a "full" compliance to entire spec)

Hendy