3x4" sheet film no longer manufactured, although it is available from some suppliers in a limited set of emulsions. Since 4x5" film is widely available in a number of emulsions, you can down your own 4x5" film down in a darkroom. Cut-film backs are readily available. Unfortunately, no Grafmatic holders were made for 3x4" cameras.
As for 120 roll-film backs, Graflex did manufacture mount some on boards for 3x4" Graphics, but none of these had the critical pin-roller assembly for ensuring film flatness.
individuals have crafted their own, but making sure that the film
is in the correct position seems like a task for an expert.
Another option is to shave down a 4x5 back so that it fits; this seems to be more easily done, and would not have film plane location problems.
The 3x4" cameras are generally a difficult option for user
cameras in modern times; consequently, they are often available
more cheaply than the 2x3" or 4x5" models, and are a good option
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