Do you want each 3D array to remain inside its cell, as it is now? If so, I think your best bet is to use the cellfun() function, which will perform an operation on each cell of an array.
You can use the reshape() function (as the first argument to cellfun).
You want to look at the documentation for these two functions, and try them out separately to see what they do, before trying them both on your problem.
That being said, here is some code that works, to give you some idea of the syntax.
new_image = cellfun(@(x)reshape(x,2592*3,4608),segmented_images,'UniformOutput',false)
BUYER BEWARE: I don't know if this actually reshapes your images the way you intend to. You'll need to do some testing to confirm the output.
