With that change to the cables, it worked! I haven't tried it in all the rooms yet, but it worked in the two rooms I was primarily concerned about, specifically the room with the fiber-router box and the small bedroom which I'm going to turn into a home office. Another day I'll try it in other rooms, and then decide if I'll buy any more of the adapters and how many. Another question: Where should I put a MoCA filter to block the signal from leaking to the outside world? Or is it unnecessary, given that it seems none of the cables are connected to anything beyond the house after I disconnected #1?

Thanks, I'll try that later.

The problem is probably with the splitters inside the house, which I can't get to, so I'm giving up on this at least for now.

After reading up about that DirecTV splitter, it appears that the #1 cable was coming from the dish, and #2 was from an SWM power inserter. #3 would carry the signal to a satellite box inside the house.