Hi Catherine, the best one is three, but I agree with Tom it's not a really natural sentence. This is because it's not really clear what speed you are referring to. How quickly they speak? How quickly they drive? If you are referring to how quickly they speak, you would say "The Japanese people speak quicker than they do.". Similarly, if you are referring to driving. Generally it's better to construct a positive comparison sentence than a negative sentence.

Of your choices, I would think three is the best, but this is a very awkward way to say this. I think one is completely incorrect. I would say "Their speed is not as fast as that of the Japanese" and I would never use any of the three that you propose. I hope that helps.