Progress bars are hard. 
I think the problem above is that you initiate loading 3 files, but those files themselves reference other files. For example, glTF can reference .bin or texture files. The LoadingManager doesn’t know about those extra resources until the first resources have come back.
You can do some “hacks” when displaying the progress bar to make this look smooth. Or, consider packing your glTF and its resources into a single GLB: https://glb-packer.glitch.me/. That may still show some extra requests (used internally to parse embedded texture objects) but should be closer to what you’d expect.