I played the trilogy back-to-back. I got ME1 and ME2 like two-three months before ME3 released. It's an excellent game and I never quite understood the absolute hatred the ending got. I do say that maybe it's because I didn't have to wait years to see the story play out, though. Ending a story is difficult. There are certainly things they could have done better, but, especially at the time people were dismissing the trilogy as a whole just because of the final moments and that never made sense to me. I thought the ending was fine. Not perfect, not even great, just fine.
Play ME3, you'll love it. Fantastic game. Plus, I imagine going into the game knowing it doesn't have the strongest ending may not even bother you as much and you may have my thought of "wait, people are this upset over that?"
EDIT: Forgot to add that playing through the trilogy back-to-back is still one of my favourite moments is gaming. Also, about going into something while knowing it doesn't have a strong ending: I did that with The Sopranos. All I heard was that the ending was controversial. Not only is it one of the greatest shows, I thought the exact quote from above, "wait, people are this upset over that!?"