if thousand tail watcher is played then get starcrossed does the trigger ability still resolve

Yes — Thousand-Tailed Watcher's "When you play me" ability still resolves even if it's bounced by Star-Crossed.

Once the trigger is on the chain, it's independent of its source — removing the Watcher doesn't fizzle it (FAQ #11231; Rule 392). Since the ability is a blanket effect (no targets), it simply applies -3 Might (min 1) to all enemy units present when it resolves, even if the Watcher is back in hand. Note it only affects units on the board at resolution — units played after won't be reduced.