It depends on how you define "distance". There are many different ways you could do that in principle. One way would be to use the element-wise p-norm: for a 3-D tensor A, this is given by 
and can be computed in MATLAB by norm(A(:),p). For example, for the 2-norm:
and can be computed in MATLAB by norm(A(:),p). For example, for the 2-norm:>> X=zeros(2,2,2);
>> X(:,:,1) = [0 2;1 3];
>> X(:,:,2) = [1 4; 5 6];
>> Y=zeros(2,2,2);
>> Y(:,:,1)= [4 3; 2 1];
>> Y(:,:,2) = [2 5; 6 2];
>> A = X-Y;
>> p = 2;
>> distance = norm(A(:),p)
distance =
6.4031
