Well, if you're planning on expanding eventually, then I would not consider buying Sony or anything except Nikon or Canon. Both companies generally have the ability to produce better cameras than Sony, especially at the high end (Nikon even uses Sony's sensors and still produces better output). Furthermore, Nikon and Canon do have more extensive lens availability for future expansion. In the long run, you'll be able to use the lenses that you buy now in 10 or 20 years without a problem, but you'll probably upgrade your DSLR in only a few years. At least for Nikon, it's easily possibly to use 40 year old lenses without significant issues (at least on the pro-level bodies, although this is much less true on the D40/60).

If, on the other hand, you plan to treat this as one purchase, it probably doesn't matter much. At the beginner level, the particular differences will not be significant. In any case, the best test is to go to a store and try out all three cameras.

Regarding Canon and Nikon, they will trade off who has better cameras at which level for the foreseeable future. Either system will provide something at some point. At the moment, it seems Canon is a bit better at the low end, and Nikon a bit better at the higher end. However, the lower end cameras handle remarkably like the higher end cameras for both companies, and if you have ideas of future upgrading, definitely play with the cameras and see which you prefer.