Hi @Lisa Warren and welcome to the Community!
It's quite important to first of all understand Jira's work type hierarchy. Tasks and Stories are at the same hierarchy level in Jira - so if you want to apply 3 hierarchy levels, these need to be:
Epic > Story > Sub-task
Make sure you have those defined as such in your csv file and also make sure all 3 work types are available as work types in the Jira space you want to import your csv file into.
In the csv file, you will have to make sure you identify each item with a unique identifier. Use those identifiers to link each child item to its parent.
With that preparation in mind, see this support article explaining the technical steps for preparing and importing a csv file. Pay special attention to the section on Creating parent / child relationships.
Hope this helps!
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