Why do I always get the selectedIndex 3 when I select 5, 10 , 15 etc.?
Also I am not clear where you get index 3, I hope you could post the complete code for us to check!
I did not want the AutoPostBack; got it to work using Updatepanel.
Through your description, I suggest you check where you bind the dropdownlist and check if set the default value.
Also I hope you could post the complete sample code for us to test!