Untitled

<html>
<select onChange="x()">
  <option selected disabled>Pilih</option>
  <option id='comboa'>a</option>
  <option id='combob'>b</option>
  <option id='comboc'>c</option>
</select>
<input type='hidden' id='texta' placeholder='a'>
<input type='hidden' id='textb' placeholder='b'>
<input type='hidden' id='textc' placeholder='c'>
<script type='text/javascript'>
  var a = document.getElementById('comboa');
  var ta = document.getElementById('texta');
  var b = document.getElementById('combob');
  var tb = document.getElementById('textb');
  var c = document.getElementById('comboc');
  var tc = document.getElementById('textc');
  function x(){
    if(a.selected == true){
        hideall();
        ta.type = "text";
    }else if(b.selected == true){
        hideall();
        tb.type = "text";
    }else if(c.selected == true){
        hideall();
        tc.type = "text";
    }
  }
  function hideall(){
    ta.type="hidden";
    tb.type="hidden";
    tc.type="hidden";
  }
</script>
</html>