http://www.linuxprinting.org/show_pr...P-DeskJet_3420
'Does not understand PCL as most other DeskJets, it uses the so-called "Lightweight Imaging Device Interface Language" (LIDIL), these printers are supported by the "hpijs" driver beginning from version 1.3. The driver supports resolutions only up to 600 dpi, head alignment settings stored in the printer are not made use of, so use a one-cartridge (CMY) printing mode in case of mis-alignments.
...
No known autodetection methods.
This information has been proofread.'
You will simply have to manually configure it.
RO.