JavaScript - Fecha valida con 3 inputs

<input type="date"/>

<h1 id="texto"></h1>

<label for="start">Start date:</label>

<input type="date" id="start" name="trip-start"

       value="2018-07-22"

       min="2018-01-01" max="2018-12-31">

<script>

  var a = document.getElementById("start");

  a.onchange = () => {

    document.getElementById('texto').innerHTML = a.value;

  }

</script>

function validarFecha() {

    var day = document.getElementById("dia").value;

    var month = document.getElementById("mes").value;

    var year = document.getElementById("ano").value;

   var fechaR;

   month=month-1;

   fechaR=new Date(year,month, day);

   return ((day ==fechaR.getDate()) && (month==fechaR.getMonth()) && (year==fechaR.getFullYear()));

<input type="number" value="30" max="31" maxlength="2" id="_dia" style="width: 50px;"/>

<input type="number" value="12" max="12" maxlength="2" id="_mes" style="width: 50px;"/>

<input type="number" value="2020" maxlength="4" id="_anio" style="width: 50px;"/>

<button id="btn">Enviar</button>

<script>

    document.getElementById("btn").addEventListener('click', () => {

        let _date = `${document.getElementById("_mes").value}/${document.getElementById("_dia").value}/${document.getElementById("_anio").value}`;

        let date = new Date(_date);

        alert(date)

    });

//Da como resultado:

//Sat Nov 27 2021 00:00:00 GMT-0500 (hora de Ecuador)

</script>