If there's a way to round exact corners of figures in FreeCAD?

Upon further thought... I'm not sure. Mental geometry is hard.

I was thinking that the radius of curvature would be constant as you rotated around the vertex. It may work:

So you could make a sphere inside your part, but don't merge it with your part, the sphere is just a tool. The radius of the sphere would be the desired radius of your fillet. The sphere would be made tangent to each of the edges that approach the vertex(the corner).

Then you remove everything outside of the sphere. To do that you could sketch a triangle where the points of the triangle land on the three edges. But the plane the triangle sits on needs to be on the vertex of your part and it is parallel to the plane that intersects the three points where the sphere meets the edges. Then extrude your triangle up to the surface of the sphere. But it needs to be a subtractive extrude, so it deletes material from your part.

Maybe I'll try. Its a fun excuse to use freecad more.

make a triangle in the corner, merge it with a sphere using a boolean cut.