It has 3 attacks (1 with the bite, and 2 with its claws) it also has 3 legendary action points per round, and 3 legendary actions to choose from (Detect, Tail attack, Wing attack(takes 2 points))
The Slow spell would reduce its 3 attacks to 1 attack per round? does it affect how many or what kind of Legendary actions it can take? such as only getting 1 Legendary action per round? or not being able to use the wing attack because it costs 2 points?
I think the key to this question resides in the line: "Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn."
My take on this is that it only affects the standard attacks, bite/claw/tail/breath weapon, the rest of what it is capable of are not affected. Legendary and Lair actions are special abilities that the creature has and would not be affected. In the case of the Tail and Breath Weapon attacks, since it can only do one of them as an action normally, Slow would not do anything here. Frightful Presence is not a melee or ranged attack, so again no effect. So the multiattack would be the only part that Slow would change making it so it could only Bite or Claw once.