I have three dropdownlists that I fill with the contents of viewbag variables. I want the content of the second variable to be filtered when I select a value of the first using JQuery or javascript. The same with the third dropdownlist, which is filtered by what is selected in the second.

function SelectedIndexChanged(accion, idnum) {
            var nuevaSelProyecto = "";
            var proyecto = "";
            var nombre = "";
            switch (idnum) {
                case "ddlSelContrato": {
                    //var getValue = document.getElementById('ddlSelContrato').selectedOptions\[0\].value;
                    var e = document.getElementById("ddlSelContrato");
                    var getValue = e.options\[e.selectedIndex\].value;
                    nombre = "Contrato, indice seleccionado: " + getValue;
                };
                    break;
                case "ddlSelProyecto":
                    nombre = "Proyecto";
                    break;
                case "ddlPtoStudio":
                    nombre = "Punto Estudio";
                default:
                    nombre = "Defecto";
                    break;
            }
            alert("Alerta, indice: " + accion + " - " + nombre);
        }

    



            Contrato  
        
        @\*--------------------Listado desplegable de Contrato--------------------------\*@
        @Html.DropDownList("ddlSelContrato", new SelectList(ViewBag.SelContrato, "Id", "Nombre"), new { Class = "ddlStyle", onchange = "SelectedIndexChanged(this.value ,id)" })
    
    

    

            Proyecto  
        
        @\*--------------------Listado desplegable de Proyecto--------------------------\*@
        @Html.DropDownList("ddlSelProyecto", new SelectList(ViewBag.SelProyecto, "Id", "Nombre"), new { Class = "ddlStyle", onchange = "SelectedIndexChanged(this.value ,id)"})
    

    

            Punto de Medicion  
        
        @\*--------------------Listado desplegable de Puntos de estudio--------------------------\*@
        @Html.DropDownList("ddlPtoStudi