Three maps in. One texture out.
X-Plane keeps metalness and roughness inside the normal map. Drop the three images you already have and get the single texture the simulator reads — no plugin, no install.
Your normal, metallic and roughness images. File names do not matter — the maps are recognised from the pixels.
The four channels become one texture, right here in the browser. Nothing is sent anywhere.
One PNG per texture set. Point your OBJ at it and add NORMAL_METALNESS so metals light up.