Navmesh Agent "move back" when player comes to close?

Something like...

 Vector3 toPlayer = player.transform.position - transform.position;
 if (Vector3.Distance(player.transform.position - transform.position) < 3) {
   Vector3 targetPosition = toPlayer.normalized * -3f;
   navMeshAgent.destination = targetPosition;
   navMeshAgent.Resume();
 }
 

This just gets a point 3 units backwards away from the player, sets the destination to that, and then tells the agent to go there.

You would really need to do more work however, for example checking to see if it's a valid location via https://docs.unity3d.com/ScriptReference/AI.NavMesh.SamplePosition.html, and if it's not using https://docs.unity3d.com/530/Documentation/ScriptReference/NavMeshAgent.FindClosestEdge.html to get the next best spot.