Based on the author's comparison of the three realms, the conclusion about the Greeks that is most clearly true is:
3. The Greeks believed that their gods could not be completely known or understood.
This conclusion is supported by the descriptions of the realms of Zeus, Aïdes, and Poseidon, each of which is associated with mystery and the unknown. The language used suggests that these realms were enveloped in mystery, indicating that the Greeks recognized a sense of uncertainty and mystery surrounding their gods and their domains.