I collect the little 3" CDs; I probably have over 100 of them. I'll have to check once I get home for an exact count. Most are singles, but I have four CD albums that came with a second 3" CD of bonus tracks (Nine Inch Nails, Meat Beat Manifesto, and a couple of Japanese imports by Lush and Belly IIRC), an album on three 3" discs (Oranges and Lemons by XTC), and 4 of them as part of a Bjork box set. The format didn't last long here in the USA during the original late 80s run, but they remained popular in Japan. They've had something of a renaissance lately, with small labels producing them again, and also the Pock-It line from Europe. The availability of 3" disc jewel cases, created for the new recordable media but not available back when they first came out, seems to have helped in their recent resurrection.
They were not part of the original red book standard, but were created about 1987 and added to the red book at that time. My first CD player from 1986 (a Technics) didn't have a recessed spot in the tray for the little discs, so I had to eyeball a mini CD onto the right spot before inserting the tray. My first 3" CD was "Don't Let's Stop" by They Might Be Giants, which I purchased new in 1987. IIRC the first one released was "Peaches en Regalia" by Frank Zappa, on Ryko also that year. I also used to have a Subaru Forester, made in Japan, with a stock slot-loading CD player that would play the little CDs.
I bought a lot of 18 or so of these on eBay several months back, and have been meaning to list the duplicates on the music barter thread here at AK.