There are two possible approaches here, and both will involve using a formula.

One is to create a conditional sum.  Essentially, create a formula, say @Cat1Cat2, that looks like:

IF {MyCategory} IN ["Category1","Category2"] THEN {MyAmount}
ELSE 0

Then, put a summary field in the Report Footer that sums @Cat1Cat2.  This summary will ignore Category3, as all of those records go to 0.  Do note that this puts your total at the end of the report, not between Category2 and Category3.


The other approach is to add another level of grouping.  In this case, make @Cat1Cat2 look like:

IF {MyCategory} IN ["Category1","Category2"] THEN "Needs Total"
ELSE "Do Not Total"

Then, create a group based on @Cat1Cat2.  You can put a summary field in the Group Footer, and suppress the Group Footer for the "Do Not Total" group.