The Psion Series 3mx was the last upgrade in the popular 16-bit (SIBO) Psion Series 3 line of PDAs.

Announced in July 1998, after the release of the 32-bit (EPOC) Psion 5, the 3mx was essentially an upgraded Series 3c. The 3mx came in two models: 1MB and 2MB.

The 3mx was identical in appearance to the 3c (63mm x 43mm x 6mm), except that the 3mx sported a matt silver metallic paint job, rather than the rubbery feel of the 3c. The 3mx also featured the same installed software applications: Agenda, Data, Jotter, Time, Calc, World, Word, Spell, Patience, Sound, Sheet, Comms, and OPL (programming) tools.

Where the Series 3mx was improved was with a faster processor -- the 3mx had a 16 bit NEC V30MX (80C86 compatible) running at 27.684 MHz, compared with the NEC V30 running at 7.68 MHz on the 3c -- and a faster RS232c connection, which was boosted to 115 kbps. For compatibility reasons the processor speed could be reduced by pressing Ctrl + CapsLock (the word "SLOW" appearing in the Status Window sidebar). Another minor difference was the size of backup battery, the 3c taking a CR1620, the 3mx a CR2025.