To be fair, ME3's ending was doomed before that game started.
I'd place it around the start of ME2.
I'd place it around the start of ME2.
In contrast, ME1's ending may have well been a master class in game cinematography and writing.
The combat wasn't very engaging either it was a boring feeling cover shooter. None of your character abilities or gear or anything seemed to have much of an effect.
Edit: Deus Ex series too
I originally asked because for most of the games I've played (and the ones you mention) the overall story played out the same regardless of your choices.
There could be branches and dialog changes but they always reconverged at important story points that stayed constant. And of course the cutscenes at very end would be different. Take DeusEx - some minor things would be different depending on your actions, and at the end you basically got to press a button to choose your ending.
The only real exception I can think of is Fallout New Vegas, where the game storyline would become wildly different as you chose to help different factions....that game was a rare exemplar of "choices matter".
My point was that ME3 was doomed because ME2 utterly failed at bridging the gap between the strong start of an epic that was ME1 and what should have been the conclusion in ME3.
In fact 'fail' may be generous. I don't think it even 'tried'.