Question: 3d interpolation

I have a 3D data-points of the form x,y,z. What is the easiest way to get the interpolated value z=f(X,Y) for given coordinates X,Y (which are of course not in the data list)? In fact I want to have a formula of the form z=f(x,y) to estimete value of "z" in the middle points of (x,y).

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