Combining images into video with ffmpeg, but from multiple folders into the same frames

I have files like so:

Trees/000.png
Trees/001.png etc

Clouds/000.png
Clouds/001.png etc

Hill.png

I want each frame of the video to be a composite of all 3 images, where Trees/000.png will join with Clouds/000.png and also have Hill.png in the background for each frame.

I could hack my way around this by doing what I know, which would be making a trees video and a clouds video, combining those together, then combining them with the hill - but I think there would be significant quality loss doing it that way so would ideally like to do it all in one go, if possible.

Any ideas?