Only one material displaying out of three

Off the bat, I’m a complete noob at this. Tear into me gently, experienced users.

I’m trying to modify a video game mesh that someone else modified first. It’s one object, one mesh, and one texture, but three materials that all draw from that one texture. For a while I was modifying it and getting in-game just fine, but recently only one material in 3 will display in-game – the other two are black. This change happened around the same time I was modifying the UV maps, and I can choose which one displays in-game by loading it in the UV editor before I export it.

I presume this can either be fixed by correcting whatever change I made that told Blender to only output one material, OR by merging all three materials into one. They all draw from the same texture anyway, so I don’t see why it was split into three materials in the first place.

How would smarter blenders fix this problem?