// Cursor position selects the left, middle,
// or right third of the display window
void setup() {
  size(100, 100);
  noStroke();
  fill(0);
}

void draw() {
  background(204);
  if (mouseX < 33) {
    rect(0, 0, 33, 100); // Left
  } else if ((mouseX >= 33) && (mouseX <= 66)) {
    rect(33, 0, 33, 100); // Middle
  } else {
    rect(66, 0, 33, 100); // Right
  }
}