The rendered grid will be a drawn object that you can manipulate and edit. Experiment with the various options to get the grid you want (or close enough), and then manually edit it by hand if needed. This is found in Extensions > Render > Grids > Isometric Grid. If your goal is to draw an object that looks like a 3D grid, use the render isometric grid extension. If you don't see the snapping tools, on the View menu, hover over Show/Hide and click Snap Controls Bar. You can use the snapping tools so that when you draw or move, it'll snap to this grid. If not already visible, on the View menu, click Page Grid.If there is an existing grid in the Defined Grids, you may want to deselect Enabled or just remove that grid. In the Document Properties on the Grids tab, in the Creation box, select Axonometric grid, and then click New.On the File menu, click Document Properties.To show the grid, on the View menu click Page Grid or press Shift + 3. Inkscape has a grid that you can used to guide your drawing, such as snapping points to it.

Wrzlprmft's answer already covers drawing something like your image, so I'll cover the other two. It's not clear from your question whether you are just trying to recreate the image in your question, draw a 3D grid, or draw something with a 3D grid for reference.

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