Super-short summary from someone who has heard the LCD-3C & LCD-X and owns the LCD-3F: (as usual, all of this is IMHO - personal experience can and will vary!)
1. Treble - Addition of the Fazor's very nearly closes the gap on the high-end that the LCD-X's had over the LCD-3C's. The Fazors give the LCD-3F's additional resolution in the upper frequencies, although with the LCD-X's it's hard to tell if this is due to them being different in construction/voicing or just the fact that they also have the Fazors! (Please realized that the increased treble is a relative thing, these are still Audeze cans and not HD-800s). WINNER = TIE
2. Bass - The LCD-X's still have a very slight edge over the LCD-3's (C or F variant) in the low end. Their is more bass, although this is truly an incremental thing - all the Audeze's kill in this area. It's not bass-head "boom-boom-boom" but it is very present, controlled and accurate. WINNER = LCD-X's
3. Mids - Here's where the LCD-3's pull away from the pack, both the C and F versions. While the X's have a very solid mid-range, I found they lacked the "magic" of the LCD-3's. This is harder to describe fully, you just have to experience it. They presence of the music, the duration of the notes, the fine detail - it's all amazing. WINNER = LCD-3's
OVERALL WINNER (again IMO) = LCD-3F's!!! I really liked the LCD-X's all around, but with the LCD-3F's it was true love (and still is).
Note 1 = I actually didn't know I was getting the LCD-3F's - when I ordered they had not announced the Fazor roll-out. I just got lucky and was one of the first to get them. They really are awesome.
Note 2 = All of these cans were driven by the same Burson Conductor, which I found did them justice all around. Amp-headphone synergy can make a real difference.
Hope that's useful!