CodePen Embed - QWNdRLw

<button>Кнопка</button>
<button>Кнопка</button>
<button>Кнопка</button>
<button>Кнопка</button>
<button>Кнопка</button>

.random {
  background: red;
}

const btns = document.querySelectorAll('button')

const getRandom = (min, max) => {
  return Math.floor(Math.random() * (max - min) + min);
}

if (btns.length > 3) {
  btns[getRandom(0, btns.length)].classList.add('random')
}

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