As I said earlier, this HOWTO doesn't cover the Series 3 ( e.g. 3/3a/3c/3mx/Sienna ), however, as I run into information, I will try to place it here anyway. This section will probably never be organized (unless someone else keeps it uptodate), but it still might contain useful information.
p3nfs, referenced above mounts the psion drives over the serial cable at /psion.stand/mnt, by default.
nfsc is a VT100 emulator from Rudolf König.
Nick Murray has created Shell3a, which is a command line interpreter which allows basic file operations as well as some sophisticated functions such as command history, filename expansion, batch file support, input and output redirection, pipe like feature :), aliases and shell variables, keyboard remapping and macros and UNIX or DOS syntax for pathnames.
PsiLin, mentioned in the Series 5* sections, also supports Series 3*
Various utilities for Series 3* from arb.
Zerlegeanleitung Psion Serie 3a, e.g. Disassembling a Series 3a (in German only)
A naked Series 3
Series 3 x-ray
Series 3 FAQ: part 1, part 2, part 3, part 4