demo https://codepen.io/gnjo/pen/zYBBzzj?editors=1010

 //プレーンを用意して、カメラを別に用意して全面に張る。
 ...
 ///////////////////////////////////////
 // 2d world
 var o2=plane2d(renderer)
 ctx2d=o2.ctx2d,update2d=o2.update2d
 ///////////////////////////////////////  
 
//レンダリングの順番を指定する
 ...
function animate(){
 requestAnimationFrame(animate)
 renderer.clear() ////for 2d world/////
 renderer.render(scene, camera)
 update3d()
 update2d() /////////2d world//////////
 draw(ctx2d) ///////
}

/////////2d world ////////////////////
function plane2d(renderer){

//レンダリングのオートクリアを外す。
 renderer.autoClear = false;////////////////

 let width=renderer.domElement.width
 ,height=renderer.domElement.height
 ,camera
 ,scene
 ,ctx
 ,canvas
 ;
 //console.log(width,height)

 canvas=document.createElement('canvas')
 canvas.width=width,canvas.height=height
 ctx=canvas.getContext('2d')
 // Create the camera and set the viewport to match the screen dimensions.
 camera = new THREE.OrthographicCamera(-width/2, width/2, height/2, -height/2, 0, 3 );
 scene = new THREE.Scene();

 // Create texture from rendered graphics.
 var tex = new THREE.Texture(canvas) 
 tex.needsUpdate = true;
 tex.minFilter =THREE.NearestFilter;

 // Create HUD material.
 var material = new THREE.MeshBasicMaterial( {map: tex} );
 material.transparent = true;

 // Create plane to render the HUD. This plane fill the whole screen.
 var planeGeometry = new THREE.PlaneBufferGeometry( width, height );
 var plane = new THREE.Mesh( planeGeometry, material );
 scene.add( plane );

 function update2d(){
  tex.needsUpdate = true;    
  renderer.render(scene, camera);  
 }
 return {ctx2d:ctx,update2d:update2d}
}
///////////////////////////////////////