I have a question about choosing a domain name for SEO?

[Dr. Sasi Attili]

I wish to buy a domain name containing matched key words. I have 3 keywords I am targeting. I do not want to go into specifics here. But say the words I am trargeting for SEO purposes are X, Y and Z. The search volume for XY, YZ and XZ individually, is quite high. However the search volume for XYZ together is 0. So my question is, would it still make sense to buy a domain name called XYZ.com or should I make a choice between XY.com, XZ.com and YZ.com.?
Thanks Mark. XYZ.com will contain 21 characters in total. Is that ok?
So guys. I am finding it difficult to chose the best answer both Mark and Abhijit seem right in their own way. However I am going for Mark as I do still believe that keywords in url still has some weightage based on my current existing website