<form class="form" id="_side3">
<label for="inlineFormInputGroupUsername2">Имя пользователя</label>
<div class="input-group mb-2 mr-sm-2">
<input type="text" class="form-control check-for-value" style="margin:0px" id="inlineFormInputGroupUsername2" placeholder="Имя пользователя">
<div class="input-group-prepend">
<div class="input-group-text">@</div>
</div>
</div>
<label for="inlineFormInputGroupUsername2">Номер</label>
<div class="input-group mb-2 mr-sm-2">
<input type="number" min="0.00" max="44418.48" step="any" class="form-control check-for-value" style="margin:0px" id="inlineFormInputGroupUsername2" placeholder="0">
<div class="input-group-prepend">
<div class="input-group-text">@</div>
</div>
</div>
<div class="form-group">
<label for="exampleFormControlSelect1">Example select</label>
<select class="form-control check-for-value" style="margin:0 auto" id="exampleFormControlSelect1">
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
<option>5</option>
</select>
</div>
<button type="submit" class="btn btn-primary" id="submit" disabled="disabled" style="background:red; border:0px" >Next</button>
</form>
<script>
_side3.onclick = () => {
const checked = _side3.querySelectorAll('элементы с классом, у которых есть значение').length
const boxes = _side3.querySelectorAll('всего элементов с классом').length
submit.disabled = (checked !== boxes)
}
</script>
const foo = Array.from(document.querySelectorAll('.someclass'))
.filter(elem => parseInt(elem.value) !== 0)