Apparently no support for H3. Quote "Although aimed primarily at current H1, H2, H2F and H4X users wishing to upgrade to H5 capabilities, the H5X can naturally act as a backup for H5D users too. For 3rd party digital backs, the H5X offers the same functionality as the H4X." H5X supports H5D capture units only apart from H1 and H2 units. (probably not Leaf). H4X does not appear to support any H backs un-naturally and unfortunately except film.