Can you lose an ending before you start playing Mass Effect 3?

Considering all the number of branches possible when starting ME3, are there any endings that are unattainable based on your imported saved game? That is, is it possible to eliminate one or more of the new endings based on your previous actions?

Solution 1:

Without getting too heavy on spoilers:

Yes.

The ending is affected by some choices in Mass Effect 2. There is no choice that allows for all endings, so there isn't a 'better' option here.

Solution 2:

Not really!

Only one choice from ME2 matters somewhat and that's whether you destroyed the Collector Base or not. What matters most is your Effective Military Strength in ME3 and of course your final choice.

In other words your ME1/ME2 decisions won't cause you to miss any endings (just a few slight variations).

Here is a full guide on the possible endings.