I have extensively used all three of these, and have taught MATLAB as part of a linear algebra course. I'd probably order them for newbies as MATLAB < Maple < Mathematica. Of course, this depends upon what you want to do with them! In my linear ranking I was thinking of matrix computations (matrices with rational or floating point entries), and since MATLAB specializes in these, it is perhaps not surprising that it is easier to learn if this is what you want to do.
Maple and Mathematica are both extremely powerful symbolic computation systems. As you probably know, both systems have passionate fans and comparisions can be inflammatory. They seem to pass the baton back and forth regarding various aspects of symbolic computation, but most would probably agree that Mathematica has generally stayed ahead in terms of producing beautiful graphics, while Maple is ahead in terms of documentation and perhaps capabibilities related to Groebner basis computations. Mathematica's grammar is undeniably awkward in some respects; most students who have been previously exposed to programming languages such as C will probably find Maple grammar easier to figure out when trouble strikes. Mathematica turns out to have undocumented functions; discovering these can be highly disconcerting since it usually happens in the context of trying to understand some utterly unexpected behaviour!
In case there are any lurkers out there who are interested in computational group theory or computational homological algebra, I'd like to mention two more specialized symbolic computation systems.
First, GAP is an amazingly powerful open source system which often offers state-of-the-art algorithms for computing with finite groups and their actions, rings, or more general sturctures, as well as finite dimensional Lie algebras and so on. No question that the GAP manual is not for the faint of heart, however!
Second, I think Macaulay2 has a lovely and elegant grammar which makes programming a real delight. The programming language in Macaulay2 is "very strongly typed", which is appropriate since it forces you to keep all the levels of structure in mind. To be fair, there are several competitors such as Singular which are probably in wider use, but Macaulay2 is worth learning just to experience its elegance.