Does Thousand-Tailed Watcher give -3 Might to the mechs that spawn from Ferrous Forerunner dying after Watcher is played?

No, Thousand-Tailed Watcher will not give -3 Might to the Mech tokens spawned by Ferrous Forerunner if the Forerunner dies after the Watcher has already been played.

According to the RiftJudge FAQ (ID 8473 and 8571) regarding Thousand-Tailed Watcher:

• Timing: The -3 Might reduction only applies to units already on the board when Thousand-Tailed Watcher's "When you play me" ability resolves.

Because the Mech tokens are created by the Deathknell ability of Ferrous Forerunner after the Watcher's ability has already resolved, they are not on the board at the time the Watcher's effect is applied. Therefore, they do not receive the -3 Might reduction.