<button class="btn">button</button>
<div class="main">
    <div class="block red active"></div>
    <div class="block black"></div>
    <div class="block green"></div>
</div>

let button = document.querySelector('.btn'),
    blocks = Array.from(document.querySelectorAll('.block')),
    pointer = 0;

button.addEventListener('click', function(){
  // Убираем прошлый блок
  blocks[pointer].classList.remove('active');

  if(pointer + 1 >= blocks.length)
    pointer = 0;
  else
    pointer++;


  // Отмечаем новый блок
  blocks[pointer].classList.add('active');
});