Equal thirds across three columns
The split always follows the 3-column layout of the profile grid: 3 tiles is one row, 6 tiles is two rows, 9 tiles is three rows. Every tile is an exact slice of the source, so a square source yields square tiles - a 1080 px image split three ways gives 360 px tiles. A non-square source yields non-square tiles, and the profile grid crops those, breaking the seams.