This is a set of four three-sided 'fudge' dice, with the special feature (for odd-face-count dice) of clearly showing the value on a flat surface, face-up. They are supplied in a cage; this reduces the cost. Each die is identical to each other, and the single three-sided 'fudge' die.
The cage is fairly easy to cut to release the dice - I used the child-safe scissors shown in the visual assembly instructions above. (Please note that the purple meeple construction team is not essential to this process, and is not supplied.)
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).