Let 'r' be the row number you want to access, 'c' be the column you want to access and 'n' be the thrid dimension you want to access. Then you can access the respective number using M(r,c,n) ; Where M is your (512,256,128) matrix.
If you want to access complete matrix, say n = 3 ; Then
M3 = M(:,:,3) ;
If you want complete first from 4'th matrix.
M(1,:,4)
Accessing elements in 3D is similar to 2D, but in 3D you have to mention the third index.
