Summary: FilletSrf is limited to handling exactly three surfaces that meet at a point. Here's a technique that will work if you have 5 surfaces to fillet.
|Use FilletSrf with Extend = no and Trim = no, to fillet all adjacent surfaces, and opposing surface combinations, except the two that are the farthest apart.|
|Trim away the portions of the spheres that are not needed using the surface edge curves with the CRV sub-object picking option. Optionally Shrink the trimmed surfaces.|
|At this point you can trim the original surfaces to match the filleted edges, or just make new surfaces from the surrounding planar curves. An easy way to do that is join all of the fillet surfaces and sphere fragments, and Cap the planar holes.|