This is a three-sided 'fudge' die, with the special feature (for odd-face-count dice) of clearly showing the value on a flat surface, face-up.
The roller is based on a Reuleaux triangle, with the corners slightly clipped to improve its rolling characteristics.
The symbols ('+', '-', '0') are sized such that the same amount of material is present in each face.
I believe that this three-sided roller form is superior to a cubic die with paired values, because an unscrupulous player able to throw a die such that it rotates on one axis can reduce their exposure to undesired values. (This is the case for all possible layouts of a d6, which stems from the fact that four is not divisible by three).
If you would like four or more copies of this die please see this listing for a set of 4.