Origin Of Triple Threat/Multi-Competitor Matches?

The first one was Shane Douglas v. Terry Funk v. Sabu in February 1994 in ECW as far I've always heard. I'm sure some other smaller indy promotion somewhere might have come up with it previously, but that's the most notable example where everyone else took it from.

That being said, I believe the concept of the Four Corners tag match precedes it. That's basically a four-way match that functions as a normal tag team match, but you can tag either opponent in. So other people came close to the concept.