The rules text for the Constrict ability: "The monster deals the listed amount of damage to any number of creatures grabbed or restrained by it. Each of those creatures can attempt a basic Fortitude save with the listed DC."Rules text for the Engulf ability: "...A creature that fails its save is pulled into the monster’s body. It is grabbed, is slowed 1, and has to hold its breath or start suffocating..."
Ergo, a creature that has engulfed a target is grabbing them and can constrict 3 times without needing to use the Grab action to maintain the grabbed status, because the grabbed status is maintained by virtue of the target being engulfed.
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