When-List-Changed Trigger on an Item -> Urgent

I wanted to know, if we can write a triiger on an item.
I have few Items and they are associated with Push button, which when clicked give me a List of Values. And they are in a hierarchical order. For Eg:
Product_legal_name is at the highest point. We can select a product_legal_name.
After that there is another list call GL class which depends on the product_legal name.
Even this has push button, which when selected gives a list.
After this we have product_type which depends upon both the above i.e., product_legal_name and GL_Class.

So, I need to write a code in such a fashion that when I select product_type the bottom
option should become ALL. When I select something in the bottom and move to top that it product_legal_name then all the bottom fields should become ALL.

So, some one has advised me to put a When List Changed trigger on each of the item.

Kindly could you suggest me, hoe to get it done.
I need urgent help on this.
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