Conditional text in group footer

I am new to Access and am trying to figure out how to set a text
control in a group footer to "A", then change it to "B" if a
condition is met somewhere in the details of the group.
The data would look like:
Group1, Group2, Group3, "A"
Group1, Group2, Group3, "B"
Group1, Group2, Group3a, "A"
Group1, Group2, Group3a, "C"

In the footer of Group3 I have a text control, when I hit the detail
record = "B" I set the control to "B". Where I have trouble is where
do I reset the text control back to "A" at the start of the next
group, so that when the next group prints the text control displays
"A" since there was no "B". I assume it is somewhere in the non-
visible header for the 3rd group, but I have no controls in there.

Thanks,
Mark