Hey there. I have a layer 3 BG set in a haunted forest. It's seen in Gloomy Gulch and Web Woods for example. You can:
-view the BG in the sample file;
-see it has a .pal and .palmask file as BG uses 6 layer 3 palettes.
-comes with 2 ExGFX files and a tilemap file. Layer 3 BG uses almost 256 8x8 tiles.
Perhaps some irrelevant information above, but who knows. Have fun!