I think it’s better to use Quads , try this
0,0-----------------------200,0
(0) (1)
| |
| |
| |
(3) ------------------------(2)
0,200 200,200
geo.vertices.push( new THREE.Vector3(0,0,0), new THREE.Vector3(200,0,0), new THREE.Vector3(200,200,0),new THREE.Vector3(0,200,0) );
geo.faces.push( new THREE.Face3( 0, 1, 3 ) );
geo.faces.push( new THREE.Face3( 1, 2, 3 ) );