How do I create a sum or running sum on a summary field in crystal reports?

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CrystalReports

I have a report that looks at turn around times for cases. Each case has multiple parts. I want to sum the times, but only for those cases with 2 or less parts.
Because the field to denote less than 3 parts is a summary field, I can't use it as the condition for a running total or other summary.
Thoughts on how to crack this?

For example,
case123 20min partA
case123 22min partB
sum of rec-call is 42

case558 20min partA
case558 20min partB
case558 20min partC
sum of rec-call is 60
I want to sum up all the totals for only those that have <3 parts. Since the sum is calculated at the group level, it won't let me choose that field.

What I have tried:

Running totals, formulas and summaries of various means