Since you are assuming that every triplet corresponds to an (x,y,z) coordinate, then just extract all the x-, y- and z-coordinate into separate vectors:
x = forces(1:3:end); % x-coordinates are 1,4,7,…
y = forces(2:3:end); % y-coordinates are 2,5,8,…
z = forces(3:3:end); % z-coordinates are 3,6,9,…
The code would need some logic to ensure that the lengths of all three vectors are the same (for the case where the user did not enter in a multiple of three values):
if length(x)~=length(y) || length(x)~=length(z)
% do some handling for case where vectors are not the same length
end
