To do it smartly and robustly, you'd check to make sure that there are the same number of zeros in each row. If there aren't you'd need to decide how to handle that, for example, take the first or last three
threeValues = find(oneRow~= 0, 3, 'first'); % Could be empty, 1, 2, or 3 at most.
if length(threeValues) == 3 .... etc.
