This one is grammatically correct:
If Einstein is correct, it would be impossible for humans to travel at the speed of light.
In the first example, the "If" suggests uncertainty so the "will" in "it will
be impossible" really sounds better if it is changed to "would".
In the third example the subjunctive mood is being used and is out of place when discussing a scientific theory as it implies wishing for a particular situation or outcome.
All three examples are understandable and the differences are subtle. The second example sounds the best to me.