for this, you need to create another object with one record. And connect this table with the existing object where the amount field is and the default value of that connection is always the first record.
Then in this new object create three Sum fields with conditions that you want.
I was wondering why my test case wasn’t working and then I finally realized that the existing test records weren’t linked to the new “totals” table. Once I updated the records it worked as expected.
Hi, I do this sort of thing too. If you want to avoid extra objects and fields (for performance reasons), you could set up three reports on a page, and filter records according to your criteria, so that you have three reports matching those criteria. Then sum the amount field.