ch1kag0,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.modalDialog{
display: none;
}
.modalDialog.open{
display: block;
}
</style>
<script>
window.addEventListener("DOMContentLoaded", function() {
var modalDialog = document.querySelectorAll(".modalDialog"), divOpen;
document.querySelector(".effectTypes").addEventListener("change", function() {
if(divOpen) divOpen.classList.remove("open");
divOpen = modalDialog[this.value]
if(divOpen) divOpen.classList.add("open");
})
});
</script>
</head>
<body>
<select class="effectTypes" >
<option disabled selected value> -- select an option -- </option>
<option value="0">1</option>
<option value="1">2</option>
<option value="2">3</option>
</select>
<div class="modalDialog">
<div>
<h2>Модальное окно1</h2>
</div>
</div>
<div class="modalDialog">
<div>
<h2>Модальное окно2</h2>
</div>
</div>
<div class="modalDialog">
<div>
<h2>Модальное окно3</h2>
</div>
</div>
</body>
</html>