- Magic in Heroes 3

The City Portal belongs to the Earth element because it teleports a hero from one point on the ground to another specific point on the ground, i.e., to a city, and not to a randomly chosen location.
The Clone belongs to the Water element because living creatures are more than half composed of water. Moreover, the material for the clone, judging by its appearance, is water.
Lightning is not fire, but an electrical discharge (a stream of electrons).
Lightning in the Air element looks quite normal because that's where we observe it.

Regarding the City Portal, it's clear what considerations the developers had in mind, but in essence, it is still similar to the Spatial Gate, as there is no difference in the mechanism of action of these spells. Both teleport the hero within the space. In this case, it doesn't matter whether specific points or random ones are used. The action is not tied to the ground; that is, it is tied, of course, but for both spells it is the same: in both cases, we start on the ground and end on the ground. But the action itself takes place in the air, and therefore it is Air Magic.
Also, regarding the Clone: it doesn't matter what creatures are made of, but it is important that they reproduce through magical action in space, i.e., a phantom of a real creature is created, which does not contain the properties of the creature itself, but only its image in space. Again, the action takes place in space and is related to air, not water.
I won't insist on Lightning because this element is complex. But lightning itself, as a cluster of electrons, causes an action similar to that of fire - that is, it ignites trees, burns organic matter, and therefore, it is still closer to fire.
IMHO