If it's repeating, it would have a little blue icon with a down arrow on it in the data source taskpane.
sum() requires you to specify the specific nodes to sum. It is not intended for summing all the children of a group. If you just want to add together three fields, you're probably best off just adding them together and not using the sum() function:
USCurrency + USCoinage + Checks
Here's your previous thread:
Getting the sum of multiple fields