When asking a new question, it is best to ask it in a new post. This allows other people who might be able to help you to see the new question.
You should not use NonlinearModelFit to estimate the parameters of a Multidimensional Gaussian distribution. You should use EstimateDistribution or something similar.
Once you've estimated the distribution you can create a 3D plot using something like:
Plot3D[PDF[MultinormalDistribution[....], x], ..........]
And you can combine plots together using the Show command.
BTW, I can't guess what your data looks like, but I'm guessing you should use the raw data with Histogram3D instead of a point plot.