formula to show data for branch

hello

having some trouble or rather logic approach

i have 3 drop down list (region, district, branch), all dependent on on another *although im having some trouble showing branch*

-i would like:
in column E *see attached* for all zip codes to show for the corresponding branches
i.e. chicago-milwaukee is a district, if i select D "chicago"...colum N *red cell - formula not working* shows all zip codes, but when all zip codes have been shown for chicago, i want it to show milwaukee

formula in column E:
{=INDEX(Zip,SMALL(IF(_Branch='Master assignment sheet'!$D15,ROW(_Branch)-ROW(BranchStart)+1),COUNTIF('Master assignment sheet'!$E$15:$E15,$E15)),3)}

that is the logic errror im having....can someone pls help!!!!