Interpolating in root

I am trying to interpolate for any given values of x,y,z for a function f(x,y,z). The problem is that there is no equation that relates f to x,y and z. I have a text file that stores a series of values for x,y,z and f. Because there is no equation , I cant use TH3F.

I could have used the TGraph3D to fill this info from a text file and then use Eval(x,y,z). But there is no such class as TGraph3D.

Is there any other way of solving this problem ?

                                        -thanks Debdatta.

If you have discrete points in a multidim distribution, you can (may be) make a parametrisation using the TMultiDimFit class.
and see corresponding tutorial at $ROOTSYS/tutorials/fit/multidimfit.C