The cube{1,2,3}.gltf files are all exactly the same model, stored differently.

cube1.gltf: embedded single file

cube2.gltf: one external buffer file (cube2.bin) that contains the concatenated
  geometry data and texture data.

cube2.gltf: multiple external files (cube3.bin for geometry, cube3_divot.png and
  cube3_normal.png for texture).
