If my Lucian has 2 Trinity Forces and 1 Skyfall of Areion, do the Trinity Forces stack or just 1 go off?

Both stack. Each Trinity Force is a separate gear with its own triggered ability ("When I hold, score 1 point"), and when you attach a gear to a unit its text is appended to that unit's rules text. So Lucian with two Trinity Forces has two independent instances of that trigger.

With Skyfall of Areion equipped, those hold effects also count as conquer effects. When you conquer with Lucian, both Trinity Forces see the conquer event and each puts its own trigger on the chain — meaning you score 2 additional points (one per Trinity Force), for 3 total (1 from the conquer itself + 2 from the two triggers). FAQ #9632 and #10436 both confirm multiple Trinity Forces stack this way.