I am new to trigger writing and I need help writing a trigger that sums 3 opportunity amount fields (Item1, Item2, Item 3) and places it in a Custom currency field called  "Total" (opportunity field) any time any of the 3 fields are populated. The user should have the ability to overwrite the custom data and I would like to add a checkbox to essentially disable the trigger if set to true. Can you anyone help me with some base code i can use to create this trigger?