In terms of performance, your question is narrowed down to the difference between the rocky and the other two, because internally the other 2 are virtually the same unit.
the ascaso looks the best (for me);
the rocky is the ugliest for me;
the rocky is the best grinder performance wise for me. Its quicker, therefore makes less noise - either because it makes less noise, or because it is grinding for less time (much faster delivery than the others);
the rocky is the heaviest most solid of the three & will probably last the other two out over a long time.
quality of grind is very similar, all grind coffee well for the type of coffee machines they are pitched at.
rocky is the easiest / quickest to adjust
other two are very slow to adjust
stepped or non stepped adjustment is the subject of endless discussion on the net but ultimately its up to the individual and if they know their stuff well enough, the stepped adjustent on a rocky shouldnt present a problem.
At RRP the rocky is the dearest, and my opinion is it is the best of the three however it is your opinion that counts when making your purchase.
Hope this helps.