Replying to Probability / Statistics question

Hopefully you guys can help me out with a fairly simple statistics problem:

Three fair dice are rolled together. Let X be the random variable denoting the sum of all three top faces. Write out the pmf and cdf.

I know what a pmf and cdf are, but all of the examples in class and other problems we've done have been done knowing what the pmf and cdf are and then computing the probabilities of X. I'm having trouble approaching this problem because it's the other way around.