What event to use when user select multiple items in checkboxlist and moves on to another checkbox

Hi,
I have 3 checkboxlists  which are dependent on each other.If a value is selected in chk1 then correspoding values should appear in chk2,chk3,
Since we want multi selection checkboxlists the selectedIndexChanged event is being called for each selection in the checkbox list control. 
But I want to make sure that the user is allowed to select multiple values in the checkbox list at  a time before page post back and when he clicks on the page somewhere outside the checkbox list control, I want the event handler to be called from the code behind file and populate other 2 checkboxlist
Can you please let me know how this can be achieved?
Thanks  a lot :)