One of the most frustrating and difficult battles in the game. The odds are severely against you here.
You start off by dropping down onto a small ledge where one of the sentinels is waiting for you (Yahim). Using a durable shield, strafe around his back and hit him 2-3 times. Try strafing counter clockwise as much as you can, as he tends to hit towards his left (your right). If you fall down before he's been taken care of, the other two sentinels will drop down and it'll be 3 on 1 (good luck!)
Once you've defeated Yahim up top, Alessia and Ricce will begin making their way towards you. Stay up on the small ledge as long as you can. There's a specific spot in the corner that gives you a good vantage point to get in a couple of hits as they jump up to the ledge. I demonstrate this in the video a couple of times.
It'll be hard to replicate exactly how my battle went in this video, so if you do happen to get knocked down or are forced to drop down, make sure you focus on damaging one specific target as much as you can so that you can eliminate the 2 on 1 advantage.
Beating these monstrosities nets you 'Soul of Sentinels' and 33,000 souls