Psion Series 3

The Psion Series 3 range of personal digital assistants were made by Psion PLC. The four main variants are the Psion Series 3 (1991), the Psion Series 3a (1993), the Psion Series 3c (1996), and the Psion Series 3mx (1998), all sized 165 by 85 by 22 millimetres (6.50 in × 3.35 in × 0.87 in). Further, a Psion Series 3a variant with factory installed software for the Russian language was called a Psion Series 3aR, and Acorn Computers sold renamed versions of the Psion Series 3 and 3a marketed as the Acorn Pocket Book and Acorn Pocket Book II.

The Psion Series 3 range was regarded in 2009 by writer Charles Stross as an unsurpassed PDA because of its long battery life (20 to 35 hours), its stable and versatile software, and its durable hardware. Others describe over twenty years of daily use with models such as the Psion 3mx. About 1.5 million Psion 3s were made. The prices were 128 kB at £179 (equivalent to £485 in 2023) and 256 kB at £249 (equivalent to £675 in 2023).

The Psion Series 3 models were a major advance on the Psion Organiser. They had an original way of managing files: the available program icons are shown in a horizontal line and the associated files display in a drop-down list beneath them. Manufacture of Psion 3s was discontinued in 1998 shortly after the launch of the Psion Series 5 (a Psion Series 4 does not exist, due to Psion's concern of tetraphobia in their Asian markets) and the Psion Siena. Psion's industrial hardware division continue to produce handhelds running the same 16-bit operating system, some 17 years after its introduction on the Psion MC range of laptops and 5 years after Psion Computer's final 32-bit EPOC PDA was released.

All Series 3 variants are powered by two AA battery cells which are easily obtainable, rather than having a specially shaped proprietary battery which are often difficult to replace. All have an internal backup battery in the form of an easily changed small button cell, which allows the main AA batteries to be changed while losing no data files. All have a DC input socket for optional external power-supply via a mains transformer AC adapter.

The Series 3's innovative clamshell design did have some problems: breakages of any of the four hinges; loss of function in the button bar between the two halves of the clam; and deterioration of the cable linking the keyboard half to the screen, leading to a serious display problem with vertical lines appearing.

Psion Series 3s have room for two flash-memory cards, which enabled backup of data. Psion, Acorn and third party software was available loaded onto such memory cards which were available as separate packs.

The Series 3 featured a tone dialing feature using a combination of its built-in loudspeaker and dedicated software for generating DTMF tones suitable for telephone systems. It could be used to dial a telephone number by holding the device to the mouthpiece of a tone dialing telephone. The tone dialing feature was integrated into the Psion's Agenda, Contacts and Data applications.

One unique feature of the Psion Series 3 software package was a built-in programming language, Organiser then Open Programming Language (OPL), which enabled users to create their own applications that ran and looked just as system programs. This, along with the rise in popularity of forums such as Compuserve and CIX, led to a significant shareware scene, (still) archived by Steve Litchfield and the 3-Lib shareware library, begun in 1994. This Psion shareware scene was mirrored a few years later by the PalmPilot shareware scene and both were forerunners of the 'app'-centric mobile market that exists today.

The Embeddable Linux Kernel Subset project has produced a small subset of Linux that runs on the Series 3a.