If you have 3 Vex Mocking, and you stun a unit at a battlefield, can you move all 3 copies of Vex Mocking to that battlefield at the same time?

To answer your question: No, you cannot move all three copies of Vex, Mocking to the battlefield simultaneously using their triggered abilities.

Here is the breakdown of why this is the case based on the rules of Riftbound:

1. Triggered Abilities and the Chain

According to Rule 376 (Triggered Abilities), when you stun an enemy unit, the ability on each copy of Vex, Mocking ("When you [Stun] an enemy unit at a battlefield, you may move me to that battlefield") triggers independently.

Because these are triggered abilities, they must be placed on the chain. When multiple triggered abilities occur simultaneously, they are placed on the chain in an order chosen by their controller.

2. Sequential Resolution

The chain resolves in LIFO (Last-In, First-Out) order.

1. You place the three triggers on the chain.
2. The first trigger (the one placed last) resolves. You choose to move the first Vex, Mocking to the battlefield.
3. The second trigger resolves. You choose to move the second Vex, Mocking to the battlefield.
4. The third trigger resolves. You choose to move the third Vex, Mocking to the battlefield.

3. Why they cannot move "at the same time"

While you can choose to move all three, they are not moving as a single, simultaneous game action. They are moving as three separate, sequential resolutions of triggered abilities.

This distinction is important because:

• State Checks: If an effect were to occur between the resolution of the first and second trigger (e.g., a reaction spell played by your opponent), the game state would be updated before the next Vex moves.
• Standard Move vs. Triggered Move: The rules for simultaneous movement (such as Rule 144.3) apply to the Standard Move action, which is a specific game action you perform during your Main Phase. Triggered abilities that move units are distinct from the Standard Move action and resolve individually as items on the chain.

Summary: You can move all three copies of Vex, Mocking to the battlefield, but they will arrive one by one as each triggered ability resolves from the chain.