The 3" discs for the Amstrad Disk Drive are called CF2 (Compact Floppy Disc). CF2 discs were produced by AMSOFT, MAXELL, Schneider, Panasonic. In late eighties, cheaper Noname discs were also available (although these were rather unreliable).
Capacity is 180 Kbytes per side (the discs can be flipped to access the other side, giving a total of 360 Kbytes per disc).
(This article, File:Amstrad Computer User8504 072.jpg, also mentions something called Hitachi HFD-2 discs.)
Pictures
Amsoft
Maxell (pack)
Maxell
Panasonic
Schneider
Schneider
Noname
