I highly doubt a new Normandy is necessary. I mean, if you look at Mass Effect so far, they have followed the trilogy archetype fairly well:
Mass Effect 1: Introduced you into the universe, discovered the threat, but didn't obliterate it.
Mass Effect 2: Radical changes take place in the second entry of any trilogy. New ship, crew, team, objective, and oh yea, you DIED.

The fight against the main threat can go two separate way: 1) You fight against a branch of the main enemy (Collectors), or-like The Empire Strikes Back- the enemy gets the jump on the good guys. Either way, it ends with the main threat open and ready to take on in the final part
Now to ME3...If it follows the trilogy archetype, that means no new team (except a few new additions) and no new ship.
......Back on topic, I don't think there will be a new Normandy