Fix cascading dropdownlists in c# asp.net

I have a set of cascading dropdown lists and I have set the autopostback to true. The dropdownlists are populated when the page starts, at least the first one with if(!ispostback). In selectedindexchanged event i populate the second and third dropdowns, but the problem is the selected index in the third dropdown. What happens is that the index is changed when I select the second and third item, but when I select the fourth, the index returns to zero. I need someone to work on teamviewer, so before accepting the project please agree to this.