This is the key for opening/to open the safe [with].

[navi tasan]

Are these sentences correct?

1) This is the key for opening the safe.
2) This is the key to open the safe.
3) This is the key to open the safe with

 

[Skrej]

Yes, they're all correct. 'With' is extraneous on #3, but not incorrect.

 

[emsr2d2]

#3 needs a closing punctuation mark.

 

[jutfrank]

The most elegant way of expressing what you (probably) mean is this:

This is the key to the safe.

The function/specific purpose of the key is perfectly clear. If you start adding words, or changing the grammar, you mess with the meaning. If you add the verb 'open', for example, you could create the possibility that there is another key, whose function is to close the safe. Adding extra prepositions, like 'for' and 'with' can be a sure way to confuse, complicate, and obscure meaning.