Melissa - initialize Rows outside of the for loop because you are just overwriting all those values from previous iterations with zeros whenever you re-instantiate it at the first line of your for loop. Try
Rows = zeros(40,3);
for ii=1:40
Rows (ii,:) = data (ii:40:120)
end
Though I'm not sure that is exactly what you will need since it is unclear on the dimensions of data.
