Diminished 7th chords are formed by adding a double-flatted 7th to a diminished triad. There are only 3 “different” dim7th chords, because all the others are simply inversions or enharmonic inversions of 3 of them.
For more information, check out playpiano.com/101-tips/15-diminished-7th-chords.htm

Tags: dim7 chord, diminished 7th chord