Currently I have information about a port for the Psion 5 and nothing about the Psion 3 series.
PLPtools is a set of libraries and utilities for enabling Unix (mainly Linux) systems to communicate with a Psion palmtop over a serial line. On Linux, a connection over IrDA, using the IrCOMM feature is also possible. A shared library encapsulates the highlevel protocol (PsionLinkProtocol) and thus makes it easy to write applications without extensive knowledge of the protocol itself. A daemon (ncpd) handles the serial connection and provides it's services on a local TCP socket.
The OpenPsion (formerly PsiLinux/Linux7k) is a project to port the unix-like operating system Linux to a small group of palmtops.