If you already know the path of the folders where the files with .nii resides then you can just use the following commands.
Please note that I have used the following script for finding .c files but it applies equally to .nii file extensions as well
% Assuming the folder paths as p1, p2 and p3
>> foldPath = {'p1','p2','p3'};
% Storing the .c files from each folder
>> k1 = dir(fullfile(foldPath{1},'*c'))
>> k2 = dir(fullfile(foldPath{2},'*c'))
>> k3 = dir(fullfile(foldPath{3},'*c'))
The obtained k1,k2,k3 variables are essentially structures which contain all the .c files residing in their respective folders.
