Melt transitions from command line
Ok, so I figured it out. Turns out you need to apply the "-mix" flag to the SECOND clip, and it will mix between the previous and current clip.


  melt test1.mp4 in=2340 out=2700 test1.mp4 in=4140 out=4500 -mix 120 -mixer luma test1.mp4 in=5220 out=5580 -mix 120 -mixer luma -blank 1500


This now works correctly, and adds a fade between the first and second clip, as well as between the second and the third.