It seems addaxis() is a bit in need of maintenance. The object handles are supposed to be stored in the userdata of the main axis, but they aren't. You can still find them though.
addaxislabels() only accepts two arguments; all others are ignored. It could be rewritten to handle extra settings, but I'm not going to do that. It would be cleaner to just dig up the appropriate axes handles and process all three in the same way instead of dealing with the main and additional axes as if they're different.
N = 1000;
time = linspace(1,24,N);
c_nacl_out = rand(1,N);
Q_nacl_in = rand(1,N);
Q_nacl_out = rand(1,N);
plot(time,c_nacl_out,'LineWidth',1.3,'Color','black')
ylim([204 207]);
addaxis(time,Q_nacl_in ,[0 6],'LineWidth',1.3,'Color','#C13BD5');
addaxis(time,Q_nacl_out,[0 6],'LineWidth',1.3,'Color','#0AA4A9');
%%%%% Axis Lable %%%%%
ylabels = {'Outlet Concentration of NaCl (g/L)', 'Inlet Volumetric Flow Rate of Brine (L/min)', 'Outlet Volumetric Flow Rate of Brine (L/min)'};
gx = gca;
gx.XLabel.String = 'Time (hours)';
hax = getaddaxisdata(gx,'axisdata');
hax = [hax{:}];
hax = [gca hax(1,:)];
for k = 1:numel(hax)
hax(k).FontName = 'Palatino Linotype';
hax(k).FontSize = 13;
hax(k).YLabel.String = ylabels{k};
end
legend(ylabels)
