How would I change my program so I turn this 2D array into a 3D one?

I am trying to code a quiz and at the moment it saves the students last score however I would like to change it so that it saves the last three. I've been made aware that I need to change the current 2d array I am using into a 3d one but I do not know how to do that.

class_number = prompt_int_big(
    "Before your score is saved, are you in class 1, 2 or 3? Press the matching number")

    filename = (str(class_number) + "txt")
    if any(name in s for s in filename):
        [name].insert(score), insert(level_of_difficulty)
    else:
        with open(filename, 'a') as f:
            f.write("\n" + str(name) + " scored " + str(score) +
                    " on difficulty level " + str(level_of_difficulty) + "\n")
        with open(filename) as f:
            lines = [line for line in f if line.strip()]
            lines.sort()