when i put the other two drops index to zero the all default to zero
This is because you are dealing with objects here (reference setting and not value setting).
See:
ddlDisabilityCover.Items.Add(lis);
ddlDreadDiseaseCover.Items.Add(lis);
You found a
ddlLifeCover.Item
which you add to other items. It's a reference type connection. Thus, in actual you are referring to one list item in all the three leading to reset on setting other.