Additive 3D UMG widget?

I need 3 monochrome UMG widgets as a texture to be used in materials. I can use a SceneCapture2D to render a single 3D widget. I thought that I could make them red, green and blue, with additive materials, and place them on top of each other so the scene capture renders them in one go. However I have no idea how to make them additive.