% if you have nodes and elements
% nodes n-by-3
% elements m-by-8
% Then
node1 = 1;
node2 = 3;
xrelative = abs(nodes(node1,1) - nodes(node2,1));
yrelative = abs(nodes(node1,2) - nodes(node2,2));
zrelative = abs(nodes(node1,3) - nodes(node2,3));
Or you can do
xyzrelative = abs(nodes(node1,:) - nodes(node2,:));
xrelative = xyzrelative(1);
yrelative = xyzrelative(2);
zrelative = xyzrelative(3);
