And here is where there is an issue: Lets say that all 3 consoles are very similar in power, graphics and control types.
What will separate them from each other is their exclusive libraries.
Nintendo will have the top brand name recognition, coming from the Wii and the massive marketshare control they have this generation. Basically, the crowd favorite. Their combination of carrying the casual market, while also preserving their lifetime zelda/metroid/mario fans will keep them in good shape.
Microsoft will bring along the FPS crowd on their systems, also with having acquired new exclusives this gen, likely to stick along with MS for exclusives in the future. Plus whatever they don't have, they can convince with money.
Sony is in a tough spot, not having the purchasing power they once did to buy exclusives, having no specific niche market except for the Playstation brand loyalists. They are going to have to really think how they can get an edge on the market, do something radical like Nintendo did with the Wii, but on their own terms.
Overall, it should prove interesting where things fall into place. It will also be interesting to see devs open up to Nintendo as they will enter the HD market, and how that affects the other consoles.