The Leica 3 viewfinder is fixed for 50mm lenses and as the title suggests, I wonder if it's possible to modify the existing viewfinder for a 35mm lens. Maybe by exchanging the front lens element of the viewfinder or by putting another lens in front of it? I just find attachable viewfinders so bulky on this otherwise beautiful Leica 3.
It would need to be a lens added to the front viewing lens - or replacing it. Undoubtedly it's possible, but getting such a lens would likely be impossible or outlandishly expensive. I think you'd need to get it made.
My simple solution is a tiny Voitlander 28-35 viewfinder, which adds almost no noticeable increase in size of these cameras. The small Leitz 50mm finder can also be a helpful supplemental impro
As far as monkeying with camera’s viewfinder, I agree with Don.
It would need to be a lens added to the front viewing lens - or replacing it. Undoubtedly it's possible, but getting such a lens would likely be impossible or outlandishly expensive. I think you'd need to get it made.
If you knew the specifics, like required focal length and curvatures, I guess you could try sourcing a lens from surplusshed.com, and somehow cut it to the right size or have a bracket that holds it in front of the existing viewfinder.
What you need to do is build a viewfinder attachment that goes in front like the "goggles" on the Leica 35mm that is used with the M3 - the M3 is limited to a 50mm finder field, so the goggles act like an extra diverging lens in front to widen the field of view. Like this: https://joerivanderkloet.com/leitz-summicron-352-0-v1-m3-goggled-review/
I've never actually used the goggled lens; it preserves rangefinder function, which you probably don't need to do for the Leica III since it has separate VF/RF. The goggle may be more than a simple diverging lens as it may have to preserve the afocal nature of the Galilean viewfinder - see the link posted by OAPOli. But you could try a diverging (concave) lens in front to start with.
I do not recommend trying to take apart the Leica III viewfinder and install different lenses to make it 35mm. You need larger lenses closer together for a wider field of view (again, see the Galilean viewfinder link), so your chance of success to damage ratio is quite small.