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The Acorn Plus 3 was a hardware module that connected independently of the Plus 1 and provided a double-density 3½ ” disc drive connected through a WD1770 drive controller and an ADFS ROM.

There were two versions of the Plus 3 produced: A Single-sided and a Double-sided drive version.

Because the WD1770 is capable of single density mode and uses the same IBM360 derived floppy disc format as the Intel 8271 found in the BBC Micro, it was also possible to run a DFS filing system with an alternate ROM, such as the P. R. E. S AP4 interface.