I want to create a (100,200,300) 3D texture, how will this look in a 2D texture (?,?)
To represent the same number of texels as that 3D texture you would need to create one very larger 2D texture, e.g. 30,000 x 200 - of course it won't look exactly the same in memory and I would think that mipmapping will be very different and probably give undesirable results. Otherwise 300 separate 100x200 2D textures?
Promit's link on texture tiling is probably what you are after here.