let b = 'white';

let c = 'white';

let m = 0;

let n = false

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background('white');

​

  // First rectangle

  if ((mouseX < width / 3) && n == false) {

    m += 1;

    n = true

  }

  else if ((mouseX > width / 3) && n == true){ 

    n = false

  }

  if (m %2 == 0){

    a = 'red';

  }

  else {

    a = 'white';

  }

  fill(a);

  rect(0, 0, width / 3, height);

​

  // Second rectangle

  if (mouseX >= width / 3 && mouseX < (2 * width) / 3) {

    b = 'red';

  } else {

    b = 'white';

  }

  fill(b);

  rect(width / 3, 0, width / 3, height);

​

  // Third rectangle

  if (mouseX >= (2 * width) / 3) {

    c = 'red';

  } else {

    c = 'white';

  }

  fill(c);

  rect((2 * width) / 3, 0, width / 3, height);

}

​