Three-way switch loops are a combination of a three-way wiring setup and a switch loop setup. In this scenario the incoming power is run to a nearby light fixture, then it branches out with three conductors leaving the light fixture and going down to one of the boxes on one end of the hallway. Then another 3 conductors leave the light fixture and go down to the other box on the other end of the hallway. The magic happens in the wiring at the light fixture box. We pass the incoming hot from the panel down to one of the white conductors going down to one of the threeway switches. It doesn’t matter which side. We have to reidentify this conductor as a hot because per code.
Next, we reidentify the white conductor going from the lightbox down to the other switch box as a black conductor. This will be one of our travelers, because it cannot be a switch leg, per code, because it’s a reidentified white conductor. Now we tie the two red travelers going to the switch boxes, together at the light fixture box. Then we do the same for the two blacks going to the switch boxes, just wirenut them together. Remember one of these blacks going to the first switch will be a normal black conductor, the other will be a white conductor that we’ve reidentified at both ends as black.
At our lightbox, this leaves us with only a single white coming from the panel, and a single black coming back from the second switch we wired. These conductors are what you’ll hook the light fixture to.