Is it possible to create a game with Unity3D that has graphics that can match assassins creed 2 3?
Sure if we talk about the graphics of the console versions then pretty much yes 
But I have doubts that you own the millions required to pay the large team of developers and artists for years to do it.
But if I had the models etc, does unity have the rendering quality to make the game that good?
There was a big thread recently (Gossip? Collaboration?) on the topic of whether Unity ‘can be used to make AAA games’, which is similar to your question. Just about every possible point of view on the subject was presented there, so I’d recommend looking for that thread and giving it a read.
For what it’s worth, the impression I get is that graphics-wise, Unity is a bit behind some of the other graphically advanced engines currently in the marketplace. However, this has to be weighed against Unity’s workflow, ease of use, price point, and portability; in other words, as with all tools it has its weak points and strong points, so you just have to choose whatever tool best meets your needs.
(Regarding the graphical limitations, I think that’s partially due to the fact that UT targets a wide range of hardware and platforms, and has to maintain both OpenGL- and Direct3D-based backends, which means that the graphical functionality in a sense represents a common denominator of all hardware, platforms, and APIs supported. That’s just the impression I get though - I could be wrong.)