There are several ways to accomplish this depending on the design/layout/structure of your form.
One simple way is by using 3 different check boxes with different names, and a numeric field to display the sum. NOTE: all fields in this example are contained within the same subform:
Check box field names (NOTE: the export value of each check box is equal to "1" when checked, "0" if unchecked):
Numeric field name:
FormCalc script on TotalField, using "calculate" event:
$ = Sum(Checkbox1,Checkbox2,Checkbox3)
this.rawValue = Checkbox1.rawValue + Checkbox2.rawValue + Checkbox3.rawValue;
As mentioned, there are several ways of accomplishing this task. The above is a basic approach.
Hope this helps.