many years ago Leitz offered the modification of the M 3 with the rangefinder of the M 4, M 4-P or M 6. But as was already mentioned this was extremely expensive like all service work they do. I think they don't offer this service any longer.
Please note that the rangefinder of the M 3 is completely different from all later types (besides this is also the reason why the M 3 rangefinder has not the flare problems of the later types). The a.m. modification means that the complete rangefinder-unit has to be replaced, not only the frames.