Tamron used to have Adaptall lenses that convert to R mount. I have read that Nikon F mount can be converted as well, but required surgery. But other than that, I can't think of any lenses that can be converted to R mount. Usually, photographers want to adapt or convert R lenses to other mounts. I have converted several of my R lenses to Sony alpha mounts.
The R3 is based on the Minolta XE. You might as well get a XE and use the older Minolta lenses.
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