I have three video tracks, all three are images, one background, two transparent pngs on top.
Then I have the same thing repeating with different images, like a slideshow, but with images on top of images.

How can I dissolve between these? Wipe uses only two tracks.. but I have a composition of tracks that make up the final screen.

I can extract the composed frame since it's just still images, but it's a pain if I ever want to change anything.

usually compositions are made for that.... COMPOSITE AND TRANSFORM can be used (changing opacity) as a fade effect. (you have to manually set the COMPOSITION TRACK... from AUTOMATIC to the correct one.)