There are older games with three bells (e.g., woodrails have them). I suspect it was a budget thing; the BOM cost was probably a basic target, and so they could afford only so much. A game like North Star or Slick Chick had more bumpers than other games, so they didn't get the third bell, whereas other games had less stuff, leaving room for the cost of the gong bell. But that's speculation.

That third bell is nice to have though.