With one more day until the episode airs, I thought I'd post a quick theory. In "The Constant," the term 'constant' refers to something (or someone, rather) who you would always be able to connect to, in both the past and present. For Desmond, the constant was Penelope. Awww, how sweet.
However, I have a feeling that the term 'variable' refers to Desmond himself, or at least people like Desmond. Faraday has continuously said that the "whatever happened happened" rule doesn't apply to Desmond, and that he doesn't have those laws and guidelines to follow. Remember when Eloise Hawking showed up in "Flashes Before Your Eyes?" Remember how nervous she was when Desmond offered to buy the ring? It's because she knew that if anyone could change the future, it would be Desmond.
Think of this mathematically now. The phrase "3x" has two components: (1) The constant, which is "3". It is no greater and no less; it has already been identified and defined, and is the same no matter which way you look at it. (2) The variable, which still has yet to be defined. In other words, when it comes to the variable, the possibilities are endless; it could be absolutely anything, and depending on what it was (or what it becomes), it will has an enormous effect not only on the constant number, but on the solution of the equation as well.
So those are my thoughts.... much like the "x" variable has not yet been defined, Desmond's actions can not in any way be planned out or guessed... he is literally the only one whose free will is still intact. Everyone else is making choices they've already made before. Weird.
What do you think? Am I crazy, or heading down the right path?