How to generate a 3D or 2D smooth function for PDF in Root?

There is a strong correlation in my PDF, so I should use a 3D smooth function as my PDF.
Unfortunately,I don’t know how to generate a 3D smooth function in PAW[I can generate 2D in PAW].
So, I try to use Root to generater a 3D smooth function.
But, I has failed. It seems that there is no 2D or 3D smooth function in Root.
Is there any way to generate a 3D smooth function in Root or in PAW ?
By the way, if there does not exist a way to generate a 2D or 3D smooth function in Root.
How can people fit well when there is a strong correlation in their PDF ?
Thank you very much ^^ !