I would like to have a humanoids with three pairs of eyes placed on the same horizontal plane like in Protheans not one below the other like Batarian. I was thinking of one pair being sensitive to our visible light, other to near infrared & third to near-ultraviolet, but it's not necessary if it doesn't make biological sense. This is predatory not prey species so binocular vision is paramount.
How to justify why the evolution decided to select for three pairs of eyes?
Animals see "visible" light via rods and cones, and humans have 3 different types of cones to see different wavelength ranges (i.e., colors) of light. Tetrachromacy is condition where an animal has four different types of cones. This fourth type of cone does not expand the visual spectrum, but can allow better discrimination of colors within it.
So.
@AndreiROM wonders why you couldn't fit all 3 sets in one pair of eyes, and the answer to that is simple: real estate. Eyes are packed full of as many rods and cones as can fit. While there's 3 (or 4) types of cones, they all sample a narrow, overlapping spectrum. If you wanted to pack two more wavelength ranges in, you'd have to cut your cell density for each spectrum to 1/3.
Or, have different pairs of eyes.
But, this creates significant problems. First and foremost, 3 independently functioning eye pairs means 3 independently functioning visual cortices in the brain. The visual cortex is a significant proportion of the brain, so replicating it a bunch is going to make things pretty screwy. Even with two eyes, our cortex has a tendency to cheat and only actually process one eye and only occasionally sample the other.
So, we cheat. One visual cortex. Slightly expanded because some additional complexity is unavoidable. But, we only use one pair of eyes at a time. That way, the cortex won't get overloaded. Since we're on completely separate wavelength ranges, having all three overlap at once wouldn't be terribly useful anyway (it would be like looking at one of those 3D eye puzzles when you haven't seen it yet). Why would they overlap? One visual cortex; this is the price we pay for not being a bobble-head.
This adds some interesting elements to the system. Under normal circumstances, one of these humanoids would only have one set of eyes open at a time, and the other two closed. This is because having unused eyes closed protects them, and it's also the simplest "off" switch. It also gives them the ability, in certain, very specific circumstances, to open multiple sets and overlay the information, if that does become advantageous. Imagine trying to code break a message that requires you to see in 3 wavelength ranges at once when you have just the one pair of eyes.
But now we're back to... why do they need three sets of eyes to begin with? Visual wavelength ranges are certainly a possibility, but you'll need to find a dire biological reason for them to need eyes that can see in those wavelength ranges. Maybe there's areas of poison gas that can only be seen in ultraviolet. Maybe their prey can't be seen at night except infrared. Anything that can only be seen in a particular range and seeing it would vastly increase either survival or mating chance will exert evolutionary pressure on developing a system to see in that wavelength range.
While they are predators, something to consider is that there are very few species that nothing kills, particularly since most species are willing to kill members of their own species. That means defensive abilities are still important. In this case, whatever the outside set of eyes, those are your "prey" eyes (the eyes that are most useful when something is hunting you). They have a much wider field of view, but much smaller binocular overlap, which means they're good at noticing things from more directions, but bad at focusing on them.
There are alternatives, most notably spatial frequency, but either way would work if you apply the appropriate evolutionary pressures.
Maybe early on in their evolutionary branch the creature had 3 sets of fairly primitive eyes. Pray species adaptation and maybe moving more subterranean made selective pressures focus on infrared for hunting but They also produce a chemical visible in ultraviolet for mating display so 2 sets of their eyes specialized in different ways.
Maybe the center eyes stayed fairly primitive until they became more surface focused. They still have the mating display and still are night hunters but the branch that becomes more humanoid also stays active for part of the day and selects for stronger human visible spectrum as well.
More eyes are useful for gaining a wider field of vision.
A great way to consider why an organism might evolve a specific trait is to look at other animals that already have that trait. For extra eyes, we have a fantastic real work example: spiders.
Spiders all have four pairs of eyes, for a total of eight. However, not all spider eyes are created equal. In hunting species, like jumping spiders, a single pair of eyes are used as the dominant visual organs for spotting prey. These eyes are larger and more developed, giving good detail of prey animals for the spider. However, the spider's main eyes have a relatively narrow field of vision, which is bad when you're fairly small and other things think you're tasty.
Spiders make up for this by having other, less developed, eyes to enhance their field of view. These additional eyes are set wider apart, which is bad for binocular vision, but great for spotting big things coming to eat you. Interestingly, researchers have found that spiders specifically react to threats when they perceive them with these additional eyes. In two eyed animals, on the other hand, eyes can be specialized for either binocular vision, like in hawks, or for a wide visual field, like in deer, but not both.
While humans evolved from two eyed animals, it's perfectly reasonable that humanoid aliens could have evolved from a smaller creature possessing more eyes. These eyes would remain useful for increasing peripheral vision as the ancestral creatures evolved into increasingly complex forms, especially for small hunters for whom both prey spotting and predator evasion were important. In an environment with lots of big predators, such as the Cretaceous Earth, additional eyes used in this way would continue to confer an evolutionary benefit, even for organisms as large as humans.
I don't know how much sense do near-infrared eyes make, from what I know snakes sense infrared with their pit organ, but if they do work then maybe Heralds evolved to hunt three distinct types of pray:
Even foraging plants that could easily be spotted only under UV could improve your survival chances, for exampleReindeer developed UV vision in the arctic. So if all those food sources provided ample calories to pay for the extra pair of eyes, and it was more advantageous to keep going after all of them, then I guess it seems plausible that some creature evolved to use them, especially if as @Tathel said its ancestor started with set of 3 relatively simple eyes.
If they're horizontally placed it's interesting to know which ones are the closest ones to the nose, I assume it's the heat vision, with ultraviolet on the outer edge.
Anyway they would make an interesting artwork.
