I have all three - and most often, I reach for Fusion because there are some cool moves in this one. Boxing is my second choice, and I hardly ever do MMA Kickboxing. For Cathe kickboxing, I prefer 4DS Kickbox, KPC or Kickmax.
None of the three MMA dvds are super advanced or tough, so if you are an advanced exerciser, you will breeze through them without a problem.
To keep my heartrate up during the workouts, I usually do jumping jacks or high knee jogging between the routines (i.e., when Cathe pauses to let the heartrate come down, or explain the next move).
All of this is not to say that I don't enjoy them. Fusion is a lot of fun, and Boxing moves quickly. They are good workouts for when you want something a little less intense and time consuming.
CCC is definitely the DVD to get if you want an intense workout!
Have fun choosing!!!