Answer : 

There are several ways to do this but by far the simplest is to "force" the guard to answer the first question truthfully, like this:

Question 1:  Will you answer this question with a 'Yes'?

No matter what answer the guard gives, it will be truthful.  Therefore the guards next two responses will definately be lies.

Question 2:  Is door 1 the safe door?

If the guard answers 'No' then you know that door 1 IS the safe door, since you know that the guard is lying.
If he answers 'Yes' then you know that door 1 is NOT the safe door and it must be either door 2 or door 3.

Question 3:  Is door 2 the safe door?

As above, if the guard answers 'No' then you know that door 2 is the one to pick.
If he answers 'Yes' then you know that door 2 is NOT the safe door so it must be door 3.