About the 4 dimension model fitting

Hi All, Now I am having problem when I trying to fitting my experimental data to one existing model. My equation has 3 independent variables theta=f(temp,solution,sample) and four fitting parameters. I am pretty sure about the range of the fitting parameters by doing the theta=f(temp, solution) for individual sample using the curve fitting toolbox. Now I am trying to fit all the data together. Is there a easy way to achieve that? Thanks sooo much!