Does Eidolon of the Great Revel trigger when you cast a spell with converted mana cost 3 or less that has an X in its cost?

Eidolon of the Great Revel triggers whenever a player casts a spell with converted mana cost (now officially called "mana value") 3 or less. The key question for spells with {X} in their cost is: what is the mana value of the spell on the stack?

According to CR 202.3, while a spell with {X} in its mana cost is on the stack, X is equal to the value chosen for it. So the mana value is calculated using that chosen value. For example, if you cast Fireball with X=1 (paying {1}{R}, total 2 mana), its mana value on the stack is 2 — which is 3 or less, so Eidolon triggers.

However, if you cast the same Fireball with X=5 (paying {5}{R}, total 6 mana), its mana value on the stack is 6 — which is greater than 3, so Eidolon does NOT trigger. The outcome depends entirely on the value chosen for X.

Concrete example: You cast Chord of Calling with X=2 by paying {G}{G}{G}{2}. Its mana value on the stack is 2+3=5, so Eidolon does not trigger. But if you cast it with X=0 paying {G}{G}{G}, its mana value is 3, and Eidolon does trigger.

In contrast, in all zones other than the stack, X is treated as 0 for mana value calculations (CR 202.3b), but that does not matter here since Eidolon checks at cast time, when the spell is on the stack.