If you know the rough form of the functional dependencies, try a multiple linear regression. You can even do that in Excel, and with only three independent variables you would only need a few parameters. Try regressing this:
F(x,y,z) = a0 + (a1 x log(x)) + (a2 x exp{a3/y}) + (a4 x exp{a5/z})
With any luck at all that will give you a reasonably good approximation while only fitting six parameters (a0..a5). YOu didn't indicate how *much* data you have, and if you need to interpolate or extrapolate, which are really important factors in selecting a method.
Other options include finding a multidimensional spline libraries (Matlab has one, I think) somewhere. Alternatively, Tilly's suggestion reminded me of the loess smoothers. Those work by considering a span of "nearby" datapoints to estimate the local shape curve. There is a multidimensional implementation built into the R programming language. The major problem with loess is that memory usage is a quadratic function (O(n2)) of the number of datapoints.
Tats
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