http://riversynths.com/product/jx-3p-midi-expansion/ the riversynth shop page which sells Laurens Schillings JX-3P add on, known as the Organix kit; you will find the most impressive improvement you can do to a JX3P at the moment: A hardware and ROM update which enables the parallel use of MIDI and the PG-200 programmer; also data transfer via MIDI now is possible.
http://www.kiwitechnics.com/patcheditor.htm The Painting With Sound Universal Programmer. A very cool hardware programmer for vintage synths - especially for Roland JX- and MKS-series. At the moment (2010) the programmer is less expensive (incl. shipping!) than a PG200 on ebay.
http://www.kiwitechnics.com/jx3pupgrade.htm The Painting with Sounds JX-3P Upgrade. This one will be the killer. It is a complete rewritten operating system for the JX3P including more modulation sources (additional envelopes and LFOs) and additional sequencer and arpeggiator functions.
http://www.sonicware.de/jx3p/index.html Page of Oliver Dahlmann, who planned the same as Laurens Schilling. Olivers page contains a lot of info on the OS. (2020: the original page does no longer exist. I relinked to the page on archive.org)