I am no expert, but I believe only the RL3Di can take film. The RM3Di is only for digital backs.
No, not true. All the R-line cameras can accept roll film holders or digital backs. The RL3di can shoot roll film holders or 4X5 sheet film or digital backs.
The R camera requires an adapter plate for the digital back to mount to the camera. A similar adapter plate is required to mount a roll film holder. The catalogue number is 080002N for the adapter plate and named "Film Holder Adapter 6X9 - N Standard".
"N" indicates the new style for the digital designed cameras, in contrast to older Arca-Swiss cameras designed for film and before digital designs were introduced. Ground glass holder frame is stamped with "N" to be sure and any adapter plates should match up to N or non-N ground glass.