You can preview the position of your 3D model in the Camera view and see what it will look like in the camera frame. You can preview your scene in OpenGL mode.
You can view your 3D objects in Render mode, but only basic settings will be used to render the 3D element. To get a fully textured and lit render, you can use Autodesk Maya. If you want to render using your 3D objects using Maya as a background process, before you can view your 3D elements in Render mode, you must add a Maya Batch Render node to your Node view and set up your system with the correct path to the rendering utility you will be using. Note that you will need to add your Maya project inside the 3D object's element folder in your Harmony scene. If you do not complete this setup, the 3D model will simply not appear—see About 3D Rendering in Harmony.
If you want to override the order of your 2D and 3D elements to follow the Composite node ordering rather than the Z-axis order, you can use the Flatten node—see Flatten Node.