As mentioned in chat, I'd normally do something like this:
- Create triangle (3 sided polygon, in Photoshop)
- Give triangle a stroke
- Create masks for corners (bottom ones 45 degrees to each side, you can hold Ctrl to make them rotate in intervals. masks in red in the image)
But looking at it again, you could actually use another triangle mask to get the same result. This would give you a very precise cut:
- Create triangle
- Give triangle a stroke
- Create another triangle on top of it, centered
- Use second triangle as mask

About how to use the mask, the quickest way I've found is to do the following:
- Create the triangle with the border and convert it to a Smart Object (this is so you don't have more than one mask for the same shape at the same time).
- Create the new triangle you want to use as mask and position it below the first one in the layers panel
- You will notice that your second triangle is basically a mask by itself - it's a color background, masked with a triangle. All you need to do is drag that mask into your first triangle.
The layers in this image are: 1) The original triangle turned into a Smart Object, 2) the new black triangle with its own mask that gives it the shape, and 3) The result when you drag your triangle 2 mask to triangle 1.
