If I use LeBlanc's legend ability to copy Thousand-Tailed Watcher, does the -3 might ability trigger?

Ruling: No, the "When you play me" ability of Thousand-Tailed Watcher will not trigger when copied by LeBlanc's legend ability.

Sequence:

• First, you play a Reflection unit token (which does not yet have any "When you play me" abilities)
• Then, the token becomes a copy of Thousand-Tailed Watcher and gains its text
• The "When you play me" ability does not trigger because it was not present on the card at the moment it was played

Nuances:

• The token gains the ability text after being played, which is too late for the trigger condition to be met
• The token will still enter the battlefield as a copy with all of Thousand-Tailed Watcher's stats and abilities, but the triggered ability simply won't fire