The Acorn System 3 added a floppy disk controller card, floppy disk drive, and disk operating system ROM, replacing the cassette interface card and cassette operating system of the System 2 machine.
The System 3 became the standard workhorse for development in the Acorn lab: the Acorn Atom has been called a cut-down version of the System 3; and it was based on the System 3 that much of the development work for the BBC Micro was done.