I accidentally made this once, but I forgot how I did it. It's 3D, and it's the same dimension of a regular cube. I know how to make the arches, but I don't know how to fill it once the arches are in place.
Edit: I'd like an ability to see it as smooth while low-poly, if it's possible. I guess a triangular pendentive is a better way to explain it :)
Any help appreciated!