Create a calculation field, result text:
FieldsBoth= Field1 & "-" & Field2 (for example)
Create a summary field = Total of Field3
On the layout create a sub-summary part, defined as summarising when sorted by FieldsBoth, and put that summary field into it, along with the Field 1 and Field 2, or FieldsBoth, whichever suits.
Get your found set and sort by FieldsBoth. You can remove the body section of the report if you want, so it only shows the sub-summary lines.
Note this assumes that records containing a's and b's are always entered as Field1= a and Field2 = b. a-b will be different from b-a.
You can actually do this without the calculation field.
Make your sub summary part "when sorted by Field 2"
Then sort your records first by field 1, then by Field 2.
The same assumptions srbsbuster has made apply to this approach as well.