Do we really need a file share witness?
In my opinion , based on version of windows you have and especially using always on configuration i would recommend having a quorum configured. In your case its File share which seems OK or else you have the option of quorum drive witness.

More on this is very neately explained by @Brentozar here


  You don't have a choice as to whether or not to have quorum: Always On
  Availability Groups are built on top of Windows Server Failover
  Clustering, which requires quorum voting to understand who the primary
  is for a given AG at a given time.
  
  I think what you might be asking is whether or not you need an
  old-school quorum drive (like a Q drive on a SAN). These days, Windows
  Server Failover Clustering (WSFC) can either use a quorum drive
  witness, OR a file share witness.
  
  Windows Server 2012 R2 includes dynamic witness capabilities: if you
  have an even number of voters (like if 1 of your 3 cluster nodes goes
  down), then Windows will automatically add in your witness in order to
  be a tiebreaker. That's why you want to configure the witness (either
  a disk or a file share) ahead of time - Windows will automatically
  manage its voting rights whenever you have node failures or network
  failures.


Also refer to another part of the answer which should help you here


  In a 3 node cluster you can setup node majority as your quorum mode
  and then you can survive the failure of a single node within the
  cluster . That is in most cases sufficient. If your cluster spans
  multiple sites you might want to fiddle with the quorum-vote weight
  depending on your failover plans