How can I make a low poly triangular pendentive? 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!
You can use circle meshes to create the exact shape in your example image, if I understand correctly.

I created circles on three faces of a standard cube, removed doubles, deleted excess vertices, filled the faces, and fixed normals: