They weren't popular by any stretch of the imagination, but they weren't limited to Amstrad either (The Spectrum didn't have a 3 inch drive until Sinclair sold the Spectrum to Amstrad).
http://en.wikipedia.org/wiki/History_of_the_floppy_disk#The_3-inch_compact_floppy_disk
Back in the day, I also saw a couple of Beebs connected to 3" drives - these were in an industrial setting but I'm not sure why they had these drives rather than the 5.25" common units - perhaps better durability?
