3D Plot with Four Variables

Hello, I have four variables (Demand, NPV_all, STD_all, STD_In) and I would like to plot them in a 3D plot. I have created the plot but for some reason I can not show the legend. Can anyone help please.

if true
  Demand   = rand(7,3); 
  NPV_all  = rand(7,3); 
  STD_all  = rand(7,3); 
  STD_In   = [0.25 .5 .75] .* ones(7,3); 
  Demand_V = Demand(:); 
  NPV_V    = NPV_all(:); 
  STD_V    = STD_All(:); 
  STD_In_V = STD_In(:); 

    h = scatter3(Demand_V,STD_V,NPV_V,markerSize,STD_In_V,'filled');

    title('Economic Impact of Production Profile')
    xlabel('Demand [MMSCFD]')
    ylabel('STD [$B]')
    zlabel('NPV [$B]')
    legend('STD = 0.25', 'STD = 0.50','STD = 0.75');
end

Answer 1

[KSSV]

Demand   = rand(7,3);
NPV_all  = rand(7,3);
STD_all  = rand(7,3);
STD_In   = [0.25 .5 .75] .* ones(7,3);
Demand_V = Demand(:);
NPV_V    = NPV_all(:);
STD_V    = STD_all(:);
STD_In_V = STD_In(:);
markerSize = 10 ;
STD = [0.25 0.50 0.75] ;
figure
hold on
for i = 1:length(STD)
    idx = abs(STD_In_V-STD(i))<=10^-3 ;
    h = scatter3(Demand_V(idx),STD_V(idx),NPV_V(idx),markerSize,STD_In_V(idx),'filled');
end
title('Economic Impact of Production Profile')
xlabel('Demand [MMSCFD]')
ylabel('STD [$B]')
zlabel('NPV [$B]')
legend('STD = 0.25', 'STD = 0.50','STD = 0.75');

[Yaser Khojah]

Thanks so much for your help and that is what I was looking for. Would it be possible to connect these dots? For example all the dots belong to 0.25?