You cannot scatter plot those. scatter plot requires 2 dimensions, but you only have one dimension.
Perhaps you want something like
hA = scatter( 1 * ones(length(A),1), A);
hold on
hB = scatter( 2 * ones(length(B),1), B);
hC = scatter( 3 * ones(length(C),1), C);
m = mean([A; B; C]);
hm = line([1 3], [m m])
legend([hA, hB, hC, hm], {'A', 'B', 'C', 'mean'});
Or perhaps you just want to use boxplot()
