One way to proceed is to write a function which accepts a vector of values for your three parameters, and returns the chi-square value computed using those parameter values. Then you use fminsearch to minimize that function. fminsearch will try different values of the parameter values and do the best it can to minimize the value of chi-square. There are no guarantees of finding the best parameter values this way, but it is a common approach when it is too hard to minimize the chi-square mathematically.
There are other methods too, but fminsearch is usually the simplest. Look here for a little more detail.
