These are currently the three active championship that has a connection to the original World Heavyweight Wrestling Championship with the NWA title being the oldest.

And yes, Impact Wrestling is there because their title came from the NWA title just like the WCW/WHC and the ECW did. Just because Impact Wrestling is a smaller promotion now does not take away this connection. The NWA is also a smaller promotion, but their title remains as the oldest spin-off of the original belt.

The Universal Championship didn't have a connection until it was unified with the WWE Championship which in tune was unified with the World Heavyweight Championship (first big gold belt) and the WCW World Heavyweight Championship. The WWE Championship is included in this as well as their title also split off from the NWA World Heavyweight Championship during the Capitol Wrestling Corporation days.

So, hypothetically, should someone were to win all three of these championships they could essentially call themselves the true World Heavyweight Wrestling Champion. But they would likely not be official as the title belt is no longer active. They could be considered the George Hackenschmidt Triple Crown Winner though.