The Triple Threat Challenge in Pokemon Snap is a request that can be completed on the Founja Jungle (Night) map. The request is made by Phil, who will say that he saw a Yanmega fly away from the area, and he wants to know if something happened there.
The request will unlock after players get the melody player, so that is a pretty strong clue that you will need to use the melody player during the request.
The first thing to do will be to get on the Swamp route in the Jungle. To do this, players will need to throw an Illumina Orb at the Crystalbloom just before the swamp to reveal some footprints. You can then scan the footprints to get onto the swamp route.
Just before entering the swamp itself, players should play the Melody which will cause a Yanmega to exit the brush to their left and fly away. When inside the swamp, players should play the Melody again. This will call the Yanmega, and an Ariados will also come out of a hole in a tree. Throw an Ilumina Orb at the Ariados. The Pokemon will begin to fight, and players can take a photo of them as they duke it out.
The shot should be focused on the Ariados, and after the expedition is over they can then hand in the Triple Threat challenge.