Use case

Return the ICE score for an issue, given three number custom fields for Impact, Confidence and Ease.

Expression

Create and configure a Smart field using this expression in Jira expression parsing mode:

(issue?.customfield_IIIII || 0) * (issue?.customfield_CCCCC || 0) * (issue?.customfield_EEEEE || 0)

Replace the IDs with the ones of your number custom fields Impact (IIIII), Confidence (CCCCC) and Ease (EEEEE).