MichaelDiteeeeeeee, медитируйте, а заодно и проведите тест и напишите результат.
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<form action="http://" name="f">
<select name="from" required size="6"> <option></option>
<option>- Suvarnabhumi International Airport</option>
<option>- Don Mueang International Airport</option>
<option>- Bangkok City</option>
<option>- Hua Hin</option>
<option>- Cha Am</option>
</select>
<select name="to" required size="6"> <option></option>
<option>- Suvarnabhumi International Airport</option>
<option>- Don Mueang International Airport</option>
<option>- Bangkok City</option>
<option>- Hua Hin</option>
<option>- Cha Am</option>
</select>
<select name="test" required size="6"> <option></option>
<option>- Suvarnabhumi International Airport</option>
<option>- Don Mueang International Airport</option>
<option>- Bangkok City</option>
<option>- Hua Hin</option>
<option>- Cha Am</option>
</select>
</form>
<script>
window.addEventListener("DOMContentLoaded", function() {
var selects = document.querySelectorAll("select"),
opt = [],
val = [];
[].forEach.call(selects, function(sel, j) {
opt[j] = sel.querySelectorAll("option");
sel.addEventListener("change", function() {
var v = [].map.call(selects, function(sel) {
return sel.value
});
[].forEach.call(selects, function(sel, k) {
sel.options.length = 0;
for (var i = 0; i < opt[k].length; i++)(!~v.indexOf(opt[k][i].text) || opt[k][i].text == v[k] || !i) && sel.options.add(opt[k][i])
})
})
})
});
</script>
</body>
</html>