Cesium_pick_terrain

var viewer = new Cesium.Viewer('cesiumContainer');

var screenSpaceEventHandler = new Cesium.ScreenSpaceEventHandler(viewer.scene.canvas);


	
//Function to add a point
function addPoint(x,y) {
    var point = viewer.entities.add({
    position : Cesium.Cartesian3.fromDegrees(x, y),
    point : {
        pixelSize : 10,
        color : Cesium.Color.YELLOW
        }
    });	
}	

screenSpaceEventHandler.setInputAction(function(click) {
    var mousePosition = viewer.camera.pickEllipsoid(click.position);
    var cartographic = Cesium.Ellipsoid.WGS84.cartesianToCartographic(mousePosition);

    var ray = viewer.camera.getPickRay(click.position);
    var position = viewer.scene.globe.pick(ray, viewer.scene, position);
    var cartographic2 = Cesium.Ellipsoid.WGS84.cartesianToCartographic(position);
    var x2 = Cesium.Math.toDegrees(cartographic2.longitude);
    var y2 = Cesium.Math.toDegrees(cartographic2.latitude);

    addPoint(x2,y2);

    console.log(x2, y2, cartographic2.height);
    console.log(y2);
    console.log(cartographic2.height);

    var x = Cesium.Math.toDegrees(cartographic.longitude);
    var y = Cesium.Math.toDegrees(cartographic.latitude);
    //console.log(x,y);
    addPoint(x,y);		
}, Cesium.ScreenSpaceEventType.LEFT_CLICK);

How to measure length between three points?
Thanks
Akshay Gore