yep, so I checked 2slot => 2 slot is 20% chance for 3 slot. And it pops up a caution window, to make SURE you want to fuse (hmm, people keep saying "dumbed down"... )
so the rates are comparable, but slightly better if you craft 2-slots first (average of 6 cards to get a two slot, with 20% chance, vs 6 attempts at 3% ~18% total). I am guessing going from 3 slot to 4 will be similar.