I've been meaning to have a go at this for a while. I have seen it done a couple of times before. As always, I don't consider this to be entirely my own work or research, but rather a collaborative effort with others in the community. Let me share with you how I did it:
Making a fanned fretboard is certainly more involved, but not too difficult. The easiest way to explain how is that you want to measure the fret locations for a different scale length on either side of the fretboard. I measured the frets for a 630mm scale on the bass side and 600mm on the treble side.
The other consideration at this point is where you want the two scales to meet, i.e. which fret will be at a right angle to the neck. I chose the 12th, but it is possible to chose the nut (zero), or any other fret if you wish. I marked out the frets on the bass side first, then positioned the ruler at the 12th fret and measured backward from there from the treble side.
When cutting the frets, I marked them as usual with a pencil, then scored with a knife. When cutting the slots with my fret slotting saw I used wooden blocks clamped in place to get them as accurate as possible.
(Another very important consideration at this point is that if you are intending to taper your neck, mark the taper first, then the fret locations along where the final limits of the fretboard will be after tapering. It will not work the other way round! I had to make the fretboard twice, but fortunately I realised before I glued it) :-)
Is this making sense so far? This leaves us with a nut and bridge which will be on quite an extreme slant. This was tricky!
So that's about it really. This is the finished guitar:
So, is it nice to play? Absolutely. I feels really natural, and not at all weird. Is it worth the effort for a 3 string? Sure, why not! Would I try it again? Yeah, probably, although there's nothing wrong with good old fashioned straight frets. This was the most fun I have had making a CBG in ages. I think its the fear factor - I spent the whole time thinking 'this might not work' :-)
Give it a go if you fancy a challenge!