In salesforce enterprise edition, if we delete a partial copy sandbox we created, is the license reclaimed so that we can reuse it?
You can either delete and create a new Sandbox or refresh it. Both will fetch up-to-date data from Production.
As far as I'm aware, the only difference between deleting and creating a new sandbox or refreshing is that you'll be able to rename and add a description if you opt to delete & recreate.
Regardless, if you have 3 licences and delete a Sandbox, Salesforce will tell you you have 2 licences in use and 1 spare.