The (top of the) Triple Splitter sits opposite the Caret Splitter. The Triple Splitter is a section of track with non-parallel rails. The large marbles make it all the way down the Triple Splitter, but the small marbles and medium marbles fall in the beginning and at the middle of the track, respectively.
November 2018: The Triple Splitter does not have a catchment for the medium marbles. They just fall down beside the Lower Zig Zag. Unfortunately, because I have built the Lower Zig Zag, building the catchment for the medium marbles will be tricky. I need to work on that sooner than later!