simulate diffusion in 3D

I want to simulate diffusion in 3D.

In 2D, diffusion in time is related to repeatedly blurring an image, I, using a Gaussian kernel, G:

    I = zeros(100,100);
    I(50,50) = 10;
    G = fspecial('gaussian', [5 5], 1.0);
    for t = 1 : 10
        I = imfilter(I, G, 'same', 'conv');
        figure(1);
        mesh(I);
    end

Now I want to do this in 3D, but instead of specifying a kernel size (e.g., [5 5 5]) and sigma (e.g., 1.0), I want to specify a time parameter (e.g., tau = t * 1e-3 [sec]) and a diffusion coefficient (e.g., 1e-1 [cm^2/sec]).

I would appreciate any suggestions. Thanks.