Actually I own both LCD-3F and LCD-X. For a personal reason maybe, I refer the LCD-3 for everyday listening. LCD-X seems to be a little bit "grainy" to me compare to the LCD-3; also the sound is a bit less smooth/silky to the 3. Though, LCD-X is much easier to drive than the 3. The differences between the LCD-X to 3 are much less than from 3 to 4.
Always wanted to get an Audeze but could never decide between the 3 and X given the small price difference. To my ears the X had way more forward vocals, was more neutral and clean and had faster bass. The 3's vocals by comparison was much more laid-back, maybe a tad more organic overall. Still can't choose up to this day, and now that the 4 has entered the picture the decision will be that much harder -_-