According to your comment, you're only interested in recurring or important characters. (I am not counting the post-gameplay ending of ME3 here, which has very obvious consequences based on choices you make in the ending sequences - you will know what you're getting into.)
To summarize without spoilers, when you ask "Are all major NPCs, by the end of the Trilogy, equally likely to have survived or been killed based on some player-controllable factors?" the answer is "Absolutely not".
In light of that, let's start with squad mates, since they tend to be more non-obvious and have more ways in which they can die...
In Mass Effect 1:
You can shoot Wrex at the beginning of the Virmire mission, but this is entirely optional.
Either Ashley or Kaiden must be left behind to die on Virmire.
All other squadmates will live through the game - note that it's possible to skip recruiting a couple of them!
In Mass Effect 2:
Every single permanent squad mate can potentially die during the Suicide Mission. This is all of: Tali, Garrus, Miranda, Jacob, Jack, Grunt, Thane, Mordin, Legion, Samara, (or Morinth, if you let her kill Samara during her loyalty mission), Zaeed, and Kasumi.
Shepard can also die during the suicide mission if fewer than two squad mates live (and if he does, you can't import that data to ME3).
In Mass Effect 3 (including previously-playable squad members who aren't playable in ME3):
Any squadmate can be killed at the final mission if your readiness is too low and you take them in your squad. EDI won't die (she's in the Normandy's computers), but her physical platform will be destroyed.
Tali will commit suicide if you don't properly handle the situation with the Geth. Legion will die here in some manner or another no matter what you do.
Mordin can die on Tuchanka - if Wrex is alive at this point, it's impossible to both cure the genophage and have Mordin live.
If you sabotage the genophage cure, Wrex will figure it out and confront you, I believe he ends up dying at this point regardless of what you do.
No matter what else you do, Thane will always end up succumbing to his illness if he has made it that far. This will happen off-camera if you don't visit him in time.
Jack will be turned by Cerberus if you don't help her on Grissom Academy, and you'll have to kill her.
Miranda will die if you don't warn her about Kai Leng.
Zaeed and Grunt can die when they show up if they weren't loyal in ME2. Kasumi maybe dies, but it's hard to tell since it looks the same even if you end up finding that she lives.
Samara can die during her mission if you don't stop her from committing suicide. Morinth won't show up there, but will end up turned into a Banshee by the end of the game.
As for non-squad mates:
Notable recurring NPCs who will die in the course of the series, at some point or another, no matter what you do:
Anderson, Udina, Illusive Man, Kai Leng, Saren, Matriarch Benezia, Nyreen, the Clone
Notable recurring NPCs who may or may not die depending on your actions:
Dr. Chakwas, Kelly Chambers, Engineers Donnelly and Daniels, Barla Von, Admiral Xen, Admiral Koris, Captain Kirrahe, Eve, Conrad, Balak, Shiala, Wreav (this one hinges solely on how long Wrex lives, you won't directly influence it), Maya Brooks
Also depending on your actions, the entire Geth race and the entire Quarian Migrant Fleet (although presumably a few Quarians elsewhere would have survived).
Notable recurring NPCs who won't die, regardless:
Joker, Hackett, Aria, Aethyta, Bailey, Engineer Adams