I think many of us here have Leica LTM cameras.
We get used to the lens mounting / dismounting routine. It involves a lot of revolutions vs a 30 degree rotation with an M lens.
We get used to the separate rangefinder and viewfinder. Is there a variable eyepiece diopter on the RF window on the III?
We get used to the rotating shutter dial on top, and the front mounted slow speed dial. Don't touch that top shutter speed dial when you shoot your picture, or you will slow down the shutter curtains. Also, don't set your shutter speed until you've wound the camera. Those of us with the IIIF RD know the joys of adjusting your flash synchronizer according to your shutter speed.
Above all, you know about trimming the film leader (or fooling around with cards of various sorts) in order to load the camera with film.
However, looking at your camera list, you have an M3, a Kiev 4 and a TLR. It seems to me that you want to sell the M3 to pay for the III.
The Leica LTM approach is very different from the Leica M approach to photography. Think: much slower, and more deliberate. With the III, you're limited to open flash also, as there is no flash synch on the camera. It's for these reasons that I don't suggest that the III be your only Leica camera.