A 3" disk could be somewhat 360ko in total, basically the same as a quite basic 5"1/4DD, except that you would have to flip side on a CPC. quite rapidly, the 3"1/2 went for DD in 720ko or HD in 1.44mo at quite lesser price per disk (the disk drive could be quite expensive on the other hand) which was more suitable for 16bit computers (those would at minimum sport 512ko of RAM most of time passed the Atari STfm release / after 1986).
Some specific moded disk drives could perform special formats that were actually used as copy protection for some games, the most (only?) known exemple being Defender of the Crown on CPC. The french developper for the CPC version [[Brice Rive]] developped this specific method and went on to become UBIsoft copy protection specialist. He produced/coded [[Defender of the Crown]], but also E.X.I.T and copy protection for various [[UBI Soft|UBIsoft]] prods on CPC.
The story of this copy protection scheme was explained at CPCrulez french forum. The method consisted of moding some specific 3" floppy disk drives models so they could format the 3" floppies into something like 200-220ko per sides... Normal unmodified disk drives wouldn't be able to format disk the same way so the datas would need more floppies to be spread on and the copy, while still possible, would ask for many more disk switch and would render the game experience far less enjoyable. Cracked version of Defender of the Crown using 3 sides instead of only 2. Un-modified disk drives would still be able to read the specially formated floppies anyway (well, most of times I guess) and the extra sectors would not all be used actually.