Add a filter to another in order to have total plane magnitude and linear phase

Hi community!

I am designing a system in which I have two low pass filters in cascade (each one has been designed with filterDesigner ). Now, I would like to add a third filter before the cascade such that the total cascade has plane magnitude and linear phase. Is there a way to do that?

Till now, I have done this:

% Creating filter objects
filter_A = dfilt.df2t(filter_A_Num, filter_A_Den);
filter_B = dfilt.df2t(filter_B_Num, filter_B_Den);

% Creating the cascade
filter_C = dfilt.cascade(filter_A, filter_B);

Thanks to anyone will answer!

VG