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.
@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.