The statblock for the Otyugh in the Monster Manual (p. 248) says:
Multiattack. The otyugh makes three attacks: one with its bite and two
with its tentacles.Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target.
Hit: 12 (2d8 + 3) piercing damage. If the target is a creature, it must
succeed on a DC 15 Constitution saving throw against disease or become
poisoned until the disease is cured. Every 24 hours that elapse, the
target must repeat the saving throw, reducing its hit point maximum by
5 (1d10) on a failure. The disease is cured on a success. The target
dies if the disease reduces its hit point maximum to 0. This reduction
to the target's hit point maximum lasts until the disease is cured.Tentacle. Melee Weapon Attack: +6 to hit, reach 10 ft., one target.
Hit: 7 (1d8 + 3) bludgeoning damage plus 4 (1d8) piercing damage. If
the target is Medium or smaller, it is grappled (escape DC 13) and
restrained until the grapple ends. The otyugh has two tentacles, each
of which can grapple one target.Tentacle Slam. The otyugh slams creatures grappled by it into each
other or a solid surface. Each creature must succeed on a DC 14
Constitution saving throw or take 10 (2d6 + 3) bludgeoning damage and
be stunned until the end of the otyugh's next turn. On a successful
save, the target takes half the bludgeoning damage and isn't stunned.
If the Otyugh starts its turn and is already grappling two PCs, can it use Tentacle Slam twice plus a Bite attack?
Best Answer
No
Tentacle Slam is not technically an attack.
In the stat block, the other actions include the phrase Melee Weapon Attack while Tentacle Slam has no similar verbiage. Tentacle Slam does not call for making an attack roll with a d20.
Tentacle Slam is an action that causes grappled creatures to make saving throws. It does not qualify as an attack for the purposes of the Otyugh's Multiattack ability.
When an attack is not an attack
This prior answer explains what counts as an attack in the game mechanics. That answer draws upon the passage from the Player's Handbook:
There are many actions that would likely be considered "attacks" in-universe, but which are not attacks mechanically. Examples include grappling and spells with save versus damage effects.
When reading a term in the rules, interpret it with the definitions in the rules. When deciding how NPCs/monsters react to something, use common sense.