CF-2 Compact Floppy Disk
The CF-2 Compact Floppy Disk also known as a three-inch disc, was one of several odd-sized floppy formats that never caught on. It was 3 inches in width, and larger in height (of the casing unit the users saw; the actual disk inside is round as usual). It was released around the same time as the 3½" disk, attempting a "VHS vs. Betamax" or "BluRay Disc vs. HD-DVD" style format war.
3" discs were mainly used in the 1980s and early 1990s. Machines which used them included:
- Various 8-bit machines by the company Amstrad (sold by Schneider in some markets):
- Amstrad CPC
- Amstrad PCW (aka "Joyce")
- Spectrum +3
- Tatung Einstein
Drives came in two variants: single-sided, single-density (typical capacity about 180 kilobytes per side, in 40 tracks) -- the drive would only read/write one side of the medium, and to access the other, you'd insert the disc the other way up -- and double-sided, double-density (capacity about 720 kilobytes per disc, in 80 tracks) -- in this case the disc would only ever be inserted one way up. Higher-density drives could read, but not (safely) write, media formatted to the lower density.
The discs used in these two sorts of drives were identical, and could all be inserted either way up. (Early on, some discs were marked as being for the DSDD drives by being labelled as CF2DD or CF2-D, but reportedly there was no difference in the underlying physical media  and they were mechanically identical.)
Many (most?) 3" discs will contain a CP/M file system.
Recovering data from 3" discs
Because this format was short-lived, getting data from a 3" drive to a modern computer can be hard. Options include:
- If you have a working machine with a 3" drive, you can use it to read the data and transfer it via some other interface the machine has. For instance, if your machine runs CP/M and has a serial (RS232) interface, you can run AUXD on the source machine and the LibDsk tools on the destination machine to transfer a disc image.
- Unfortunately, some models of the Amstrad PCW in particular didn't come with any standard interfaces. There were add-ons such as the CPS8256 to give serial and parallel interfaces, or the LocoLink cable to connect to a PC's parallel port (which came with software to convert LocoScript files).
- An alternative is to connect a more common drive, such as a 3.5" drive, to your working 3" machine (alongside the 3" one). Frank van Empel has a guide for the Amstrad PCW. (another guide) Obviously you'll need the ability to read 3.5" discs.
- Or you could fit a Gotek USB floppy emulator. References for Amstrad PCW:  (Italian, with technical detail)  
- If you have only a 3" drive, it's usually possible to interface it to a PC (if the PC is old enough to have a floppy controller). Frank Van Empel has some details for Amstrad drives. Then you can use disc imaging software on the PC to recover the data (the PC's operating system is unlikely to understand the data structure natively).
The 3" drive might require maintenance first -- a common problem is that the drive belt tends to stretch. Here is a guide to renovating a drive (aimed at the Amstrad drives).