My normal atmas for Fell Cleave, or anything, are: Razed Ruins, Voracious Violet. I vary the third depending on how I feel because they all have proven to be about equal in terms of producing the result I want. Third is typically Stout Arm or Apocalypse.
These give me averages from 900-4K (hit a 5K 2 nights ago, the last time I did a Fell Cleave) depending on berserk up down, my general laziness in switching to WS set, Might Strikes up/down, mobs running out of range due to positioning issues or type of mobs (some of them stay put, others tend to swarm...)
What are you planning on cleaving?