legend plot of subgroup lines

I am preparing a gui that gives the operator the flexibility to fit n datafiles. Each datafiles contains 3 replicates. I would like to add a legend to the plot that groups the 3Xn lines and I can't get there. Thank you!

This is my code:

 % code
figure(2)
hold on;
tot_f=1;

for k=1:3:(size(Data_dim,2))
    color_plot(tot_f,:)=[rand(1),rand(1),rand(1)];
    h{tot_f}=plot(time, Y(:,k:k+2),'Color', color_plot(tot_f,:));
    xlabel('Time (s)') 
    ylabel('NU')
    ax=gca;
    ax.YLim=[0 50];
    ax.XLim=[0 100];
    tot_f=tot_f+1;
end
hold off;

legend(????);

Answer 1

[Walter Roberson]

    % code
  figure(2)
  hold on;
  tot_f=1;

    for k=1:3:(size(Data_dim,2))
        color_plot(tot_f,:)=[rand(1),rand(1),rand(1)];
        h{tot_f}=plot(time, Y(:,k:k+2),'Color', color_plot(tot_f,:));
        desired_legends{tot_f} = ... some appropriate string
        xlabel('Time (s)') 
        ylabel('NU')
        ax=gca;
        ax.YLim=[0 50];
        ax.XLim=[0 100];
        tot_f=tot_f+1;
    end
    hold off;

    group_handles = cellfun(@(V) V(1), h);   %get first out of each group
    legend(group_handles, desired_legends)