Particularly with an old-technology camera such as a IIIc, you'll have to practice a bit to learn the differences between what the viewfinder sees and what gets on the film.
At distances close to infinity, you'll probably find that the film shows at least as much as you saw through the finder. They designed it that way as a safety device -- if you see something in the finder, you can be sure it will be on the film. You can crop out any excess when you print or scan.
At the closest distances (3 to 5 feet or so) you'll encounter the phenomenon of "parallax" -- the difference between what the lens sees and what the finder sees. On a IIIc, the finder is just about exactly above the lens, so some of what you see along the top of the finder may be cut off in the picture. Experienced users of old-time RFs would learn to deal with this when shooting close-up portraits and such -- they'd compose the shot in the finder, then tip the camera upward a bit. Or, just frame all your shots somewhat "loose" and crop out the excess later.
It's problems such as this that led rangefinder camera makers to improve the viewfinder system in cameras made later than the IIIc. Although it's a cool camera, you really owe it to yourself someday to at least try a model with a more modern finder system: an M-series Leica, or a Bessa R-series model, for example. Among other conveniences, these have white framelines projected into the finder field; the lines show the area of the picture much more precisely (although still not quite as exactly as an SLR) and they move as you focus the lens to compensate for the parallax effect.
