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

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

if (btns.length > 3) {
  btns.eq(getRandom(0, btns.length)).css('background', 'red')
}