The first guardians used what element they discovered with the light. In time, some learned to master multiple elements. Our guardian happens to be "the guardian savior" so we are strong and smart enough to master all 3 elements. Shin Malphur was said to be the first golden gun hunter, and one of his titles is "the man with the golden gun" so we can assume he hasnt learned to use the other elements or hes just mastered the golden gun. Sunbreaker titans formed an order and became mercenaries. Nightstalker hunters are very rare, people like Tevis are some of the only ones who truly mastered the element. In short, most guardians only know how to use one element. Others prefer to use one element but have learned the others. Our guardian is powerful enough to switch at will depending on the situation.