This is an echo of how the ancients had seen the world, consisting of three basic, visible elements: sky, sea, and earth. They all distribute water, and thus weather.
It also well corresponds to the old Celtic view of the elements, seen as the fundamental triad.
Zeus was, among other things, the ruler of lightning and giver of the rain.
Poseidon had, under his rule, the sea, rivers, streams and lakes.
Before entering the realm of Hades you had to pass at least Styx, one of many underground rivers, and we know there are many underground streams.
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Let's consider Zeus as the personification and deification of sky-water, Poseidon - above-ground-water and Hades - underground-water.
It is obvious how important water is to every living creature. We know through science of meteorology and others, how the great water cycle is consistently performed on Earth.
Could the ancients had a glimpse of this primary function of our planet?
Glorification of weather and water-deities had always been significant in ancient agrarian societies. The triple aspect even suggests a trip farther back in time when 'three' was of the goddess(es), connected to the element of water itself, of course. After the time of transition from matriarchy to patriarchy Zeus, Poseidon and Hades may be seen as the masculine aspect of water, which itself has three states: liquid, solid and gas. Three different aspect of the same divine component, a 'watery being', controlling the atmosphere.
The atmosphere is of course regulated by the cycle of water, and has direct influence on the weather conditions, for which, especially both - Zeus and Poseidon, were prayed to.
Ancient Greeks were rather unaware of this process, nevertheless, it strangely fits into our modern-day scientific explanation. The weather still remains completely out of human control, being somewhat – godsend.
The trident, characteristic to Neptune/Poseidon and Hades/Devil, is also a metaphorical indication of the underlying unity of the three Gods.
In very simplified, symbolic scheme the personified water-cycle of Zeus-Hades-Poseidon, looks like this:
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| Water Cycle Zeus-Hades-Poseidon |
From the undeground streams (Hades) the rivers spring and flow to the sea (Poseidon). There, some of the water particles evaporates, creating clouds (Zeus). They eventually bring rain on the ground, which goes under it. And the cycle starts again.
