I just bought an MM3 the other day. I got it to use on my bigger system (~150L), because using my Barley Crusher was getting a bit laborious. I went for the 3 roller option because I always crush twice and it meant that I could get the same job done with just one pass through the mill.
The Barley crusher is great for smaller batches (in fact it was doing quite well on the larger batches too, I am just lazy!), and if I was only doing ~20L at a time there's no way I would bother going beyond a 2 roller mill. Having said that, a lot of people do - as QldKev said, the 3 roller mill has advantages regardless of batch size, but you just need to decide whether those advantages are enough to justify the extra cost. For me, its so easy just to chuck 5kg through a 2-roller mill twice, if the double crush is what you want. Its not so easy to do this with a 25kg grain bill.