SQL 2014 SP2 CU1 - AlwaysOn Cluster trouble synching to one node
SPID -3 is a deferred transaction or BOL Definition.


  A deferred transaction is one where the transaction could not be recovered fully because of an I/O error or a gross file system problem like an unavailable file.


Check your disk subsystem and run a CHECKDB in your database.

To kill the negative spid, use 

SELECT DISTINCT 'KILL ''' + CONVERT(VARCHAR(100),request_owner_guid) + ''';'
FROM   sys.dm_tran_locks
WHERE  request_session_id = -3
AND    database_id = DB_ID('Your_DBName');