How can one render/print multiple cameras from different scenes at once?

I’m working on a 2D game that is supposed to have different layers/cameras.

Though, each one of them has a different scene (the main game has his own, the background has his own and so does the UI). I’d need to actually render all those at once but I have no idea how and Google seems not to help me.

If not possible, which are the alternatives?

The usual way to do this would be to use Sorting Layers. Use one for the main game, one for the background and one for the UI (though, if you use the OnGUI() function, the UI will be on top of everything else anyway).

See for the official tutorial on this.