DOS

Thats right, but to clarify from my earlier post, the low limit screw should be set with chain on small ring in front and big ring in back to ensure cage is positioned properly relative to the chain when on small ring. There are no "steps" per se for the derailluer. For double or triple, FDs are pretty simple mechanisms that slide inward and outward as cable tension changes; triple FDs have bigger cages needed to work properly with the granny gear but the larger cage does not affect performance when the derailleur is used with a double crank; the limit screws determine how far in board (Low) and out board (high) the cage will slide. Since you say your's is shifting harshly mostly when down shift, my best guess was you had low limit screw dialed down too much so derailleur was prevented from moving inboard far enough to shift cleanly (too much cable tension also will do that). If you set the limit screws while chain was on big ring, you should redo. Otherwise, tweak cable tension until derailleur is functioning properly.

By the way, when you swapped cranks, did you change the BB to one with a spindle length compatible with new cranks? If you are still using the BB that came with triple crankset, with the double, your chainline will be off, which also could affect shifting.