How many 'amounts' are there? If there are a large number, you might want to reconsider the "flat" part and use a child table so that there are many records, each one with one "amount"
Then your total could be Sum(Child::amount) and you'd be done with it.
If there are only 2-3 'amounts', simply make your calculation field to be = amount1 + amount2 + amount3
Nothing wrong with Ninja's suggestion, but you can also define a summary field to do this and place it in a footer or trailing grand summary to get a total at the bottom of your layout/report.