Final answer:
Hermione does not levitate a Christmas tree in the 3 Broomsticks in the original Harry Potter books or films. The levitation spell she is known for correcting Ron on is 'Wingardium Leviosa'.
Step-by-step explanation:
The spell Hermione uses to levitate the Christmas tree in the 3 Broomsticks to hide herself, Ron, and Harry from view is not directly mentioned in the Harry Potter books or films. This scenario appears to be a fan-invented situation or a misremembering of a scene, as there is no specific instance in the original material where Hermione levitates a Christmas tree for the purpose of hiding in the Three Broomsticks Inn. In the Harry Potter series, the spell for levitation is 'Wingardium Leviosa', which Hermione famously corrects Ron's pronunciation of in their first year during Professor Flitwick’s Charms class at Hogwarts. However, without a specific reference to the scene mentioned, it's not possible to state which spell she used, if any, for this particular event.