Three panels is the Instagram carousel shape. Post a wide photo as one square and it shrinks; post it as three slides that swipe into each other and the whole thing fills the screen.
That is what this preset is set up for. One row, three columns, cut left to right in posting order. A 3000 × 1000 panorama becomes three 1000 × 1000 px panels, each one close to square. Turn on 4:5 padding and each panel is placed on a 1080 × 1350 px canvas with a background of your choice, so Instagram shows what you exported instead of cropping into it.
Three is also the triptych number in print. A living-room wall gets three canvases with a finger of space between them, and the picture reads across the gap. Split at 1:3 for that, print each panel at full resolution, and leave the mounting gap to the frames rather than the file.
If you want a stacked cut instead of a side-by-side one, the axis toggle turns three columns into three rows. Same three files, rotated ninety degrees in intent: a long infographic cut into three postable strips rather than a wide photo cut into three panels.
Nothing is uploaded. The split runs on a canvas inside the page, which is why it finishes as fast as your machine can draw and why a photo you would rather not hand to a server stays on your machine.