You think it might be a strength of light thing?
Like our character is so strong in the light, we can harness all three subclasses with ease.
However, Zavala, Cayde, and Ikora, strong as they are, do not have the strength of light to be able to manage, nor contain three subclasses, so they can only handle 2.
We’ve heard plenty of times that Saint has tremendous light, and perhaps it is due to that, that he can wield all three subclasses as we can, he is unique in the sense that his light is remarkable.
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Strength in Light would certainly have an effect, as well as skill and practice, but you assume too much in thinking Saint is the only one that can use all 3. Osiris probably has the strongest Light we’ve ever seen, and Ikora probably has stronger Light than Saint as well. Ikora is a master in Void and Arc (Arc being very difficult to master for Warlocks), and its very likely she can use Solar, too. Osiris can probably use Void as well. Cayde has strong Light but only uses Solar, which is only because it’s his style and preference.
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I believe that Cayde didnt use Arc cause he didnt want to.
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If I remember correctly the third subclasses (Nightstalker, Sunbreaker & Stormcaller) were all dangerous and difficult to wield for various reasons in D1. Nightstalker was difficult to control and we gained the ability off of a dead nightstalker that we had to find. Stormcaller had to harness and control the power of an Arc storm. And Sunbreakers had to prove themselves worthy of joining an old splinter faction of Titans that had left the city and were living on Mercury. So you might be right that all three can only be wielded all at once by extremely powerful guardians.