2. Don’t Augment Your Troubles or Diminish Your Chances
Here is a fun little Compendium of Nasty Augmented and Diminished Intervals. View at your own risk.
Mozart — who was a very good speller, by the way — wrote this textbook-case of a correct diminished third:
W.A. Mozart Missa Brevis No. 7 in C major, K. 258
The E-flat is just the plain-old third scale degree in the key of C Minor, and the C-sharp leads smoothly up by half step to the D, per guideline #1 above; also, the C-sharp is a secondary leading tone to D, the ii chord in C minor. (Mozart did not always write his clefs — and who can blame him? He was a busy guy, and he didn’t have Noteflight!)