Label the guards 1 2 and 3.  Ask 2 if guard 1 is a liar.  If he says yes, ask guard 3 where the exit is.  If the says no, ask guard 1 where the exit is.

I think those count as non-tricky questions

(1) Ask the person which one of the others would lie to me. The liar would point out someone else, and you can then use the person not picked to ask the second question to. The truthful person will pick the liar, but since there's only one, you will be guaranteed someone who tells the truth if you pick the unchosen one.

(2) Since the person you are asking this question to is guaranteed to tell the truth, you ask this person which gate will lead you out safely.

Keep trying: (hint: think of the one that everybody knows, and find a way to reduce the riddle)

Hm....
Ask each of them what 8+8 is.
Ask whoever says 16 which gate is correct

Overly complicated?  What about the guy who kills people who ask tricky questions

If the first guy you ask is a liar, he will say that yes, he will kill you if you ask him a tricky question (he will be lying).  But you will also get the same response if the first guard is the one who kills people.  If the answer is yes, all you know is that the guard you just talked to is not the guard who tells the truth (but doesn't kill people), you still don't know where the liar is or where the guy is that kills people

1 Question: You can ask each of them "Where is the correct door?". It's not a tricky question, so you're safe.
2 of the guards will pick the same door. One guard will pick another door. The correct door is the one that 2 guards pointed to because there are 2 truthtellers.