The Leica M3 is a 35 mm rangefinder camera by Ernst Leitz GmbH (now Leica Camera AG), introduced in 1954. It was a new starting point for Leitz, which until then had only produced screw-mount Leica cameras that were incremental improvements to its original Leica (Ur-Leica). The M3 introduced several features to the Leica, among them the combination of viewfinder and rangefinder in one bright window, the bayonet lens mount, and the rapid film advance lever. It was the most successful model of the M series, with over 220,000 units sold by the time production of the M3 model ended in 1966.
The M3 has an exceptionally bright viewfinder when compared to any previous or subsequent model, including the modern M9. The M3 has a high magnification factor of 0.92×, which is useful in critical focusing, and especially with long lenses (subsequent Leicas would use 0.85×, 0.72× or 0.58×).
Here's a video of photographer KingJvpes shooting on the streets of San Francisco with a Leica M2: