There is an old set of scripts that draw various geometric patterns but drawing a triangle is not an option. You really don't need a script for this, though.
Since you're creating a template, you can draw the triangle using the path tool or you can just hold the "Shift" key when using the paint bush tool to draw the straight lines that make up the sides of your triangle. Show and snap to the grid to help you keep things lined up.
I did a quick search for a set of dingbats that contain basic geometric patterns but surprisingly, I didn't see any. Considering how useful a set of basic shapes in a dingbat would be, I'd have guessed those would be readily available, but maybe not.
Just remember that since you'll be printing the final image, you will need to create it at a high enough resolution to look good after you print it out. Otherwise, the printer/driver will try to scale up the image to the fit the resolution of the printer and the final printout will be pixillated and fuzzy. The screen is only 75dpi but the printer is much higher so make your image large.
