The two primary commands for filleting surfaces in Rhino are:
FilletEdge:
Creates a tangent surface between multiple polysurface edges (joined) with optional varying radius values, trims the original faces, and joins the resulting surfaces together.
Is not limited to just two surfaces
Can fill in corners between adjacent fillets
Is limited to exactly three surfaces meeting at a point
The radiuses used can not be so large that they overlap each other and completely consume any surface they are following.
FilletSrf:
Creates a tangent surface between two surface edges (joined or not) with a constant radius, and optionally trims and/or extends the original surfaces.
Here's an example sent in by Mitch Heynick. Mitch writes: Fillet all vertical edges plus the base with a radius of 10. Fillet the horizontal inside edges near the top at 5. --Mitch
Here's another example sent in by Mitch Heynick. I'll admit, this one stumped me but Pascal figured it out. Mitch writes: Fillet all the surfaces except bottom with a constant radius of 5. --Mitch