Here is your data, plotted:
You have what appears to be a relationship where the dominant thing happening is a curve in (x,y,z). Think of it as a path through space, with a cloud of noise around that path.
NO. You CANNOT use polyfitn here. There is insufficient information to build a viable model, of the form z(x,y). Note that z(x,y) would be a surface, defined everywhere. To try to infer the coefficients of such a model would be essentially impossible to do well. If you try to use polyfitn, the first thing you would then do is send me a plaintive e-mail, asking why polyfitn does not do well on your data! Similarly, do NOT use a tool like gridfit. It too will fail to work well here, as your data is clustered fairly tightly around this path in the (x,y,z) space.
Hey, I'm just trying to forestall the inevitable plaintive e-mails!
Again, you have what appears to be a 3-d space curve , with noise that varies along the path, sometimes called heteroscedastic noise. Yes, it hurts my eyes just to try to type the word. But it just means that the noise distribution varies spatially, so the variance changes with location.
Next, it appears as if you have a problem with errors in all three variables. Nonlinear errors in variables problems can be nasty.
The real question is, what will you do with the resulting model? Are you looking for a nice smooth curve that goes through the middle of that banana shaped blob, just for a plot? Are you looking for predictive ability of some sort? Or are you really hoping for some sort of surface? (If the latter, abandon all hope, ye who enter here. You will need to get better data for that.)