The lattice used in Lattice Deformation Path Effect is 5x5. How could I use a, for example, 3x3 grid?

accordingly with gemini, it's not possible. that is his response when asked about:

Inkonic

The "Lattice Deformation" path effect in Inkscape does not offer direct controls for setting the number of rows and columns of the deformation grid. Instead, the grid's resolution is determined by the complexity of the selected object and the bounding box of the path.

 

However, you can influence the density of the deformation points by:

 

• Subdividing the Object's Path:

  Before applying the "Lattice Deformation" effect, you can add more nodes to your object's path. This can be done by selecting the object, switching to the "Edit paths by nodes" tool (N), selecting a segment, and clicking the "Insert new nodes into selected segments" icon in the toolbar. More nodes will result in a more detailed deformation grid, offering finer control.
• Using the "Add Nodes" Extension:

  For more precise control over node distribution, you can use the "Extensions > Modify Path > Add Nodes" extension. This allows you to add nodes based on a specified number or segment length.

Once the "Lattice Deformation" effect is applied, you can manipulate the deformation points (nodes) of the blue grid that appears over your object using the "Edit paths by nodes" tool. Dragging these nodes will warp and distort your object according to the lattice.