3 x 3 pixel art help

Hello - not sure if this is even the right place to ask these types of questions but here we go :

I'm trying to create Pixel Art using a 3 x 3 square pencil brush on a large canvas.  I have a created a 3 pixel Grid.  The Grid is visible.  I have turned on Snap to Grid.  But the pencil tool does not 'snap' to the grid properly.  I would like to create pencil strokes in 3 pixel increments but it still lets me glide the pencil through the grid lines.  Please see the image below for clarity.

Thank you in advance.