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I think wht you want is located in >View>Show> Show more options,
or Show> (Options)
In Preferences>Guides, Grids,Slices... set Gridline every: You can also set the subdivisions within the grid.
Or you can use the Polygon Shape tool. Set Sides to 3. Lower shape layer opacity and CTRL+T to adjust shape to the image.
View attachment 14444
HOLD IT.... forgot this part....
CTRL+click vector mask thumbnail in the shape layer to make triangle selection.
Make image layer active , use selection tool , right click.....