If then scenarios to highlight multiple dropdown boxes

You can use this code as the custom Validation script of the drop-down:

var DDAfields = ["Dropdown8", "Dropdown15", "dropdown 18"];
for (var i in DDAfields) {
	var f = this.getField(DDAfields[i]);
	f.fillColor = (event.value=="DDA") ? color.yellow : color.white;
}

I used the names you provided, but be aware they must match the actual field names EXACTLY for it to work, including lower-/upper-case letters, spaces, etc.