seems clear one has to take three letters out of the 16 solutions - in each case I have 2 the same and the first one different. These don't seem to be "clashes" though - and the instructions seem to suggest replacing them with a single letter ...
Prospero
Rather than thinking of removing 3 letters, try replacing 4 letters with 1. And look carefully at what 4 sequential letters in all of the modifies clues have in common.