Well, you can’t have a 7-sided polyhedron where all the faces are the same; that requires an even number. But you could make a die if it doesn’t conform to those rules:
- Let some faces be different. A pentagonal prism has 7 faces, five rectangles and two pentagons, and then they need to be just the right size to make it fair.
- Add faces that don’t count. You could make something based on a heptagonal pyramid or prism, but that won’t balance on the bases.
It looks like people have tried 7-sided dice based on both these options – for instance here and here. The 3-sided die would be more or less a non-polyhedral equivalent of the second approach.
