how to append values in dropdownlist box using another dropdownlist

how to append values in dropdownlist box using another dropdownlist

I have 3 textboxes.I need to append values according to previous dropdownlist .I have retrieved val for 1st textbox from json file(autocomplete) I have some values in 2nd field , I need to append values to 3RD field according to 2nd field.. I have retrieved values. But need to append to dropdownlist. I need to get the length of retrieved values.(count the retrieved values) I am getting undefined. please check the updated link.. and screenshot.  https://plnkr.co/edit/C6ODswCppxH6uZlOtjCB?p=preview