I use Matlab software, you can fit data to polynomials or a given function with several parameters, it fits constants in equations and calculates regression parameters. The description of which function to use can be found in the last article I published, there is an example for a function with 2 variables and 3 parameters.

You can implement a nonlinear curve fit in Excel pretty easily. 1) Pick the equation you want to fit to the data, which contains the unknown fitting parameters, 2) make educated guesses for the parameters, 3) use the parameters and the fit equation to make a prediction, 4) subtract the prediction from the real experimental data and square this difference, 5) sum up the squared differences, and 6) use Solver to minimize this sum by changing the parameters you originally guessed.

You can use my software free. Create a table of x,y,z,... in Excel, copy to clipboard, Alt-tab over to CurveFit, push Read Clipboard, push Fit Curve, push Copy Fit to Clipboard, alt-tab over to Excel, alt-F11 to open up VBA, press ctl-V to paste the function into Excel.