This three sided stitch is a very simple border stitch that has triangles formed with straight stitches. This is more common in pulled embroidery.
Bring the needle up at 1.
The needle goes down at 2 and comes back up at 3.
Repeat the previous step again. The needle goes down at 2 and comes back up at 3.
Finish triangle by going down at 1.
The next triangle will be upside down.
Continue forming triangles.