DOE for Excel 2003

I need to create a DOE for a project I am working on at work. Due to program security and accessability, I cannot download any programs, so I need to be able to do this all on Excel.

Essentially, I have a target number to reach. There are three variables which affect this number. Based on the amounts of each of these variables, my target number changes. Therefore, I will need to experiment by changing one variable at a time while leaving the other two variables constant. Once I have established several data points which result in various numbers, I need to be able to create an equation from this data which allows me to plug in an amount for one of the variables and returns the amounts of the other two variables necessary to reach my target number. Thus, regardless of what amount I choose for variable 1, my equation will produce the other amounts of variables 2 and 3 such that my target number is achieved.

Any ideas of how I might begin this process? I know Excel has these capabilities, but I am stumped. Help is greatly appreciated!!