You are sending tasks to three persons in parallel and using “wait for the first response” task option. When the first person responses (approves/rejects) the task, the task will be completed.
You can create an additional workflow for the particular workflow tasks list. Trigger this workflow automatically when the task item is changed. Check whether the “Task Outcome” is Approved or Rejected.
If it is rejected or approved, send email to this workflow task “Created By” user (Email Current Item:Created By).
When the task process workflow for an item is triggered, a workflow task will be automatically created in the particular workflow tasks list and the task is created by the workflow Initiator. If you have set the task process workflow to be run automatically when an item is created, the "Created By" user of the workflow task will be the workflow Initiator. If the workflow is run manually, the initiator is the user who runs it.
In the email, mention this task is Rejected/Approved by “Modified By” user ([%Current Item:Modified By%]).