Can a P6 Global Change be designed to be an activity counter?

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I'm designing a new layout that I want to display only the first three activities under a WBS value.  I'm looking to use a global change to flag the first three activities and a layout filter to display those three activities.

A manager wants to see a layout that displays the first three milestones for each project in a multiple project layout.  I can set something up manually, but I am currently trying to get a GC to count the activities and stop when it reaches 3 (or continue for all activities under a WBS value, either way).  Then the layout filter will display the activities that have a value of 1, 2, or 3.  I'm using the activity counter field to help with the GC, but it puts the same value for each activity.