multiply the mesh in your scene by placing it in a 'for loop', for threejs_playGnd

multiplyMesh.js

        			/* ------------------- INSTRUCTIONS -----

				to multiply your mesh
				place it in a for(){} 'loop'

				and place the for(){} 'loop' inside
				of your setup() function

				if you want to create more than 300
				or less than 300, change the number

				-------------------------------------- */

				for ( var i = 0; i < 300; i ++ ) {
					// your mesh code (from the geometry GUI) goes here
				}

				/* -------------------------------------- 
		       		cube geometry EXAMPLE bleow,

		       		NOTICE: he color property is set to 'Math.random() * 0xffffff'
		       		        this will generate a random color for each of the 500 cubes

		        	NOTICE: the mesh's position and rotation are also set to random 
		        	        variables, because otherwise the cubes would all render in 
		        	        the same spot.
		        	-------------------------------------- */

				for ( var i = 0; i < 300; i ++ ) {
					geometry = new THREE.CubeGeometry( 25, 25, 25 );
					material = new THREE.MeshBasicMaterial( { color: Math.random()*0xffffff } );
					mesh = new THREE.Mesh( geometry, material);
					mesh.position.x = Math.random() * 1000 - 500;
					mesh.position.y = Math.random() * 1000 - 500;
					mesh.position.z = Math.random() * 1000 - 500;
					mesh.rotation.x = Math.random() * 2 * Math.PI;
					mesh.rotation.y = Math.random() * 2 * Math.PI;
					mesh.rotation.z = Math.random() * 2 * Math.PI;
					scene.add( mesh );
				}

				/* -------------------------------------- 
		       		try moving the camera around the scene
		       		place the following into your draw()
		        	-------------------------------------- */

				camera.position.z = Math.sin( Date.now() * 0.002 ) * 500;
				camera.position.y = Math.sin( Date.now() * 0.002 ) * 300;
				camera.lookAt(mesh.position);

				/* -------------------------------------- 
		       		try also leaving trails
		       		replace the 'renderer = new ...' on line 27
				in the setup() function, with the code below
		       		-------------------------------------- */
		       		renderer = new THREE.WebGLRenderer( { preserveDrawingBuffer: true } );
				renderer.autoClearColor = false;

You can reuse the geometry for this snippet (less memory usage, better practice):

geometry = new THREE.CubeGeometry( 25, 25, 25 );

for ( var i = 0; i < 300; i ++ ) {
    material = new THREE.MeshBasicMaterial( { color: Math.random()*0xffffff } );
    mesh = new THREE.Mesh( geometry, material);
    mesh.position.x = Math.random() * 1000 - 500;
    mesh.position.y = Math.random() * 1000 - 500;
    mesh.position.z = Math.random() * 1000 - 500;
    mesh.rotation.x = Math.random() * 2 * Math.PI;
    mesh.rotation.y = Math.random() * 2 * Math.PI;
    mesh.rotation.z = Math.random() * 2 * Math.PI;
    scene.add( mesh );
}