http://www.youtube.com/user/iGetsLucky
This is one of quite a few channels with LEAKED (as in, not allowed) ME3 content. I'm unsure if this is against any type of rule on MMO-C, if so, please just edit that link out or something. As I see it, it's a spolier. If it was illegal, it would have been taken down (that channel has content from November, so it's quite old).
Any information you can find is based on unfinished content. I've yet to see a clip of ME3 that doesn't have bugs so obvious it's hilarious. Have this in mind if you feel it might spoil your experience playing the actual game. Whole scenes without textures missing, NPC's freezed in messed up possitions and situations, lips not moving while talking, no sounds from random sources (like weapons, hearing only one person speaking in the background in an obvious dialogue, etc etc), the list goes on.
It is however a great source if you can treat unfinished material for what it is, and/or have specific questions you'd like answered before buying the game, whatever they might be. It shouldn't have to be said that leaked information on an unfinished product doesn't properly reflect the actual finished product.