1. Open the first subreport in design view.

2. If you don't see a Form Footer section, then click Form Header/Footer on
the View menu. (You can set the Visible property of this section to No if
you don't want to see it.)

3. In the Form Footer, add a text box, and set properties like this:
   Control Source        = - Sum([strEmailSent])
   Format                   General Number
   Name                     txtEmailSentCount
This assumes your subform has a yes/no field named strEmailSent, and you
want to count the number of Yeses.

4. Test. Save. Close. Repeat for other subforms.

5. Open the main form in design view.

6. Add a text box with Control Source of:
   =[Subform1].[Form]![txtEmailSentCount]

7. Test. You can then add a plus sign, and similar expressions that refer to
the other subforms.

This will produce #Error if your subforms have no records to match, and they
are set up such that you cannot add new records. In this case, the Detail
section goes completely blank, and referring to the non-existent controls in
the subform generates an error. More info about this situation:
   http://allenbrowne.com/casu-20.html