Hi Rudl - re: fillet your side surfaces are kind of a bit lumpy and have a kind of tight bend in a very small area right near the corner here:



If I turn on reflective metallic style lighting (under Options > View > Lighting options), and zoom in a ways on that corner you can see the shaded areas which indicate that there is some tight curvature nearby there:



That kind of bunching and tight curvature in a small area is a kind of bad quality surface and filleting is pretty sensitive to such things.

It's fairly easy to get bad surface quality like that when surfaces have areas that are collapsing together, which is what happens if you build a 3 sided surface as in this case.

If you separate that surface out to be an individual surface and turn on its surface control points, you'll see they look like this:



So notice there that all the control points are kind of collapsing down to a sort of "pole" area and bunching together in lines like this:



That kind of bunching is good to avoid if possible, it is just too easy for there to be irregularities and things like little lumps and bumps in those collapsed zones.

lyess' version just happened to be proportioned evenly enough to avoid quite as bad of problems in that area.

Instead of building a surface bounded by 3 curves (which by its nature forces one spot to be collapsed down), I'd instead recommend making an initial surface that has a 4-sided boundary which can then be much more regularly shaped without any bunching or bad areas in it.

That would go something like this - here I have a profile curve and generate a sweep from it:





Then to produce the final outline you draw in some curves from the side view like this:



And then use it to trim the surface like this:



That will make 2 fragments and the top part can be deleted to leave you with this shape:



Now if I turn on surface control points for this shape notice how they are formed:



So see there how the surface control points are much more regular and not bunched together in any awkward way? That helps to make a much higher quality surface that won't have little weird folded spots in it. Surfaces like this will be handled much better by other processes such as filleting and also offsetting or shelling.

So usually it's better to build surfaces in this way by making are more regular layout to the initial surface and then trim it to get the 3-sided boundary rather than trying to construct the surface directly to a 3 sided boundary right in its initial construction.

- Michael