Like drop 2 but you drop the third note from the top instead of the second. This creates an even wider spread between the bottom note and the rest of the voicing. Drop 3 voicings have a distinctive open quality — the gap in the middle gives the sound room to breathe.
Take close position [1, 2, 3, 4 from bottom]. Drop note 2 (3rd from top) down an octave. Result: [2-12, 1, 3, 4].
When you want a wider sound than drop 2. The left hand plays the dropped note while the right hand covers the upper three notes. Particularly effective in ballad settings where you want space and resonance between the voices.