on the 303 you can hit reverse and it's like scratching because it doesnt
reset the sample. it reverses it on the spot, and goes back to normal, just like a back scratch.
on the 606, you can use the reverse effect knob to get sort of the same effect, just twiddle knob 3 and it will sound like quick little scratches.
the 404 is hard to scratch on. but if you can get a good scratch going on
anything, old fruity loops has fruity scratcher, also try wave travellor for programming your own scratches. once u got your scratches, throw some 1 shot samples over it to give it more flavor like chicka chicka, (what)
or just sample some good scratching, and chop it up.