They can, but they only can channel one type of light at a time. In D1 when your Hunter unlocks void, Cayde tells you it's a rare power and that not all hunters can do it. So all Guardian's have the potential to unlock all sub-classes, not all Guardian's can get attuned to all types light. So yes a guardian could use all three, one after the other. But I think the cool down you get when you switch is to simulate you channeling a "different type" of light. I think normally a guardian must let the light flow and charge their supper at the in game rate, but in an emergency they can activate their supper instantly, but maybe at a reduced capacity, or it requires intense concentration as they must continuously channel the light. So in the D2 tower screen Zevala had his supper charged. He's a striker, so it was arc energy. The attack happens and he needs a ward of dawn. He has to dump all his arc light and channel void so he can get the ward up. Once up, he's forced to maintain it and is completely immobile because he's channeling the light to keep it going. If he stops channeling light the ward fails, and everyone dies. For Cayde, he's a gun slinger so his golden gun was charged. But when he got knocked down he needs something. He decides to challenge light to get the knives out. But when he does this the supper is at a fraction of it's potential because he didn't let the light flow naturally and charge him. I clearly spent too much time thinking about this writing it. 😀