Uhh...I am going to go with just "triangular file"? Is this a trick questions? At least it appears triangular if I squint enough and take my glasses off.
As said above it is a scraper. Hard like a file and sharp on the three corners where the cutting action takes place. The flats should be concave to allow only the edge to contact. It can be used in all of the above suggestions. Think of it as each corner being a file tooth and only using one tooth at a time.