How do I perform nonlinear regression

I have 60 data pairs. Each data pair include 3 inputs and 1 output. The trends (2 dimension) between output and individual inputs are complex, which are not easily presented by a formula. I want to use nonlinear regression to find the relationship between inputs and output.

For nonlinear regression, normally I can use "mdl = fitnlm(tbl,modelfun,beta0)" to do regression. The fitnlm attempts to find values of the parameters in modelfun.

My question is: if I don't know the formula of modelfun, how can I use fitnlm.

Are there any other methods to be used?

Thank you.