Why do R lenses come in 2/3-CAM or ROM mounts?

Single cam

The original Leicaflex Standard camera, released in 1964, used a cam arrangement inside the lens mount to mechanically communicate the aperture information from each lens to the camera body (similar to the way the Nikon AI mount does it). As there was only one cam, ergo… "single cam".

2-cam

When the Leicaflex SL, with its integrated TTL light meter, was introduced in 1968, a new lens cam was added for aperture indexing. To maintain backward compatibility with older Leicaflex cameras, new lenses were released with two cams, so they could aperture-index on both old and new Leicaflex bodies.

3-cam

In 1976 Leica released the R3, the first of its R series SLR cameras designed in collaboration with Minolta. Because the new camera(s) had auto exposure features, this meant adding another cam, bringing the total to three. This third cam is a black stepped affair, located next to and just inside the chrome 2nd cam.

R-only

An added complication is that some newer R lenses are sold as 3rd cam only. These will only mount on R bodies and are easy to distinguish as they have a collar around the base of the stainless steel lens mount flange, to prevent you from mounting them onto older Leicaflex bodies.