There is no grid tool in Affinity Photo but grids exist, they can't be printed though. You can add a workaround by using the grids via the view menu and then using the pen tool to add a curve or two to match the grid lines and these can then be merged with entire document and then printed. The vector grid can also be saved to the assets panel or symbols (merge the curves via the geometry / layer commands). The grid is 3 x 3 because that was a request from a viewer but you can always just create the grid, document width / 3 or height / 3 etc or 10 or 20 etc
By Andrew Buckle. Photoshop custom shapes, Illustrator symbols, plug-ins, brushes, styles, picture tubes, gradients and many more for Photoshop and Photoshop Elements and Affinity Photo, Affinity Designer, Krita, Paint Shop Pro and Painter.. as well as fonts, patterns, textures etc for many others. Programming as well as designs and arts and scrap booking and a little more