Или так...
<div class='yellow' data-mount='3'></div>
<div class='yellow' data-mount='1'></div>
<div class='yellow' data-mount='2'></div>
<script>
const yellow = document.querySelectorAll('.yellow')
const v = Math.min(...[...yellow].map(o => +o.dataset.mount))
alert(v)
</script>