David,
>... Can I put the older not-K3 Ultrachrome matte black Epson ink into my refillable Epson 3880 cartridges and expect it to be the same as the K3 matte black ink?
Almost certainly yes.
> Also, can I put the HDR matte black ink into the same matte black cartridge for the 3880?
Also almost certainly yes.
Paul