Logic Puzzle!

You know that one logic puzzle with the labrynth guards? Well this is a copypasta of that with a twist, enjoy.

Three guards stand infront of three doors, one is the exit to the building, if you pick the wrong door you are stabbed, One always tells the truth, one always lies, and the 3rd stabs you if you ask questions more than 7 words long, and if you say more than 3 sentences.  What do?

>>15-19
What you are saying is true, but when I say something like "B either said 'A is a knight,' 'A is a knave' or 'A is the spy,'" that's just me giving you incomplete information and has nothing to do with what B could or could not say. In fact, you shouldn't be able to find out who the knight is.

>> 20
Oh, and lastly, since B and C did say "knight", by deduction they cannot be the knight.
No. In principle, it's perfectly possible that no one at all said "knight". For example, if A is the spy, B is the knave, and C is the knight, each could have said the other is a knave. Unless you can exclude that possibility for some other reason.

The key to solving this puzzle is to consider that (methodology spoiler, not the answer) if you had complete information of what each defendant said, you should be able to find the spy like the judge did. This is only true for a few combinations!