How do you define the top 3? If it's based on a summary of some data (sum, count, etc.), search in the Crystal help for "Top N" and follow the instructions there. If it's not based on a summary, try this:
1. Sort the data descending on the field you're looking at. This sort will happen inside whatever groups you already have defined.
2. Create a Running Total (I'll call this {#RecCount}):
Field to Summarize: Some key field in the details.
Type of Summary: Count.
Evaluate: Every Record.
Reset: On change of group - whatever the inner-most group is that the details are based on.
3. Go to the Section Expert. Click on the formula button to the right of the Suppress checkbox (DO NOT check the checkbox!).
4. Enter the following formula:
{#RecCount} > 3
This should give you just the top three records in the group.
-Dell