The S3 finder is really very intresting for a number of reasons:
1. You can see from the parallax markings that parallax is not a huge deal, especially for 35 and 50mm lenses. (In part, that's because the S3 doesn't focus as close as a Leica ... 0.9 meters versus 0.7 meters).
2. The field of view is considerably larger than the 35mm outer frame. The full field of view, if you don't wear glasses, approximates a 25mm lens -- at 1:1 lifesize magnification. I wear glasses, so I find it easier to frame a 25mm with the accessory finder, but it's good to know I can omit the finder if I'm switching lenses in a hurry.
3. The finder shows the relationship between different lenses. You can see the difference between 35mm and 50mm. If you mentally add the same distance outside the 35mm frame (fairly easy to do) you have precise framing for a 25mm lens. I use this finder all the time with a 28mm lens and, by shooting tight and pushing a bit past the 35mm frames, I can accurately frame the 28mm, even though I wear glasses. I can't see the whole frame at once, but the life-size magnification allows me to concentrate on the quadrant or third of the image where framing is critical.
4. If you look inside the 105mm frameline, there's a tiny transparent triangle that shows the parallax for the lens at close focus. This also happens to coincide accurately with the corner of the field of view of a 135mm lens at infinity. At close focus, the opposite corner of the 135mm lens coincides with the opposite corner of the 105 at infinity. I've checked this against an SP with parallax-corrected framelines. This, combined with the lifesize finder, makes it easy to accurately frame a 135 telephoto with the S3 -- as accurately as any rangefinder camera.
5. You can shoot the 85mm lens accurately by using the 105mm framelines and shooting a bit loose -- the 50mm frame helps keep you in check so you can judge the correct relationship. At close focus, where the 85mm shines in portraits, the upper left corner of the 105mm infinity frame is an accurate anchor for framing.
The SP is a somewhat easier camera to use for telephoto work. But in my humble opinion, the above factors make the S3 a better all-around camera for the wide-angle photography that is so common with rangefinders, while still being fully capable of framing all three telephotos. You can, without accessory finders, accurately frame lenses from 25mm to 135mm. And the lifesize finder makes telephotos shine. If you can see it, you can photograph it.
The whole reason I shoot interchangeable-lens cameras is so I can switch lenses. I regularly carry three lenses -- the 28/3.5, the 50/1.4 and the 35/1.8 for low-light conditions. For a vacation kit, I'll add the 105mm and maybe the 25mm. If I want to take full photographic advantage of a situation, I trade the 105mm for a combination of 85 and 135mm lenses.
