Вставить число в свободный input

const inputs = document.querySelectorAll('input[type="number"]'), 
  maxIndex = inputs.length - 1; 
let currentIndex = 0; 
 
[...document.getElementsByTagName('button')].forEach(b => b.addEventListener('click', e => { 
  const val = +e.target.textContent; 
  inputs[currentIndex].value = val; 
  currentIndex = currentIndex === maxIndex ? 0 : currentIndex + 1; 
}))

<button>1</button><button>2</button><button>3</button><button>4</button><button>5</button><button>6</button><button>7</button><button>8</button><button>9</button> 
<hr /> 
<input type="number" /><input type="number" /><input type="number" />